diff --git a/lib/typeprof/core/ast/sig_decl.rb b/lib/typeprof/core/ast/sig_decl.rb
index 886b34b8..c20f50e0 100644
--- a/lib/typeprof/core/ast/sig_decl.rb
+++ b/lib/typeprof/core/ast/sig_decl.rb
@@ -21,14 +21,21 @@ def initialize(raw_decl, lenv)
         end.compact
         # TODO?: param.variance, param.unchecked, param.upper_bound
         @params = raw_decl.type_params.map {|param| param.name }
+        @params_default_types = raw_decl.type_params.map do |param|
+          ty = param.default_type
+          ty ? AST.create_rbs_type(ty, lenv) : nil
+        end
       end
 
-      attr_reader :cpath, :members, :params
+      attr_reader :cpath, :members, :params, :params_default_types
 
-      def subnodes = { members: }
+      def subnodes = { members:, params_default_types: }
       def attrs = { cpath:, params: }
 
       def define0(genv)
+        @params_default_types.each do |ty|
+          ty&.define(genv)
+        end
         @members.each do |member|
           member.define(genv)
         end
@@ -46,6 +53,9 @@ def define_copy(genv)
       def undefine0(genv)
         mod = genv.resolve_cpath(@cpath)
         mod.remove_module_decl(genv, self)
+        @params_default_types.each do |ty|
+          ty&.undefine(genv)
+        end
         @members.each do |member|
           member.undefine(genv)
         end
diff --git a/lib/typeprof/core/ast/sig_type.rb b/lib/typeprof/core/ast/sig_type.rb
index c400d916..aa0c896f 100644
--- a/lib/typeprof/core/ast/sig_type.rb
+++ b/lib/typeprof/core/ast/sig_type.rb
@@ -33,7 +33,7 @@ def self.typecheck_for_prepended_modules(genv, changes, a_ty, f_mod, f_args, sub
         if prep_decl.is_a?(AST::SigPrependNode) && prep_mod.type_params
           prep_ty = genv.get_instance_type(prep_mod, prep_decl.args, changes, {}, a_ty)
         else
-          type_params = prep_mod.type_params.map {|ty_param| Source.new() } # TODO: better support
+          type_params = prep_mod.type_params.map {|(_name, _default_ty)| Source.new() } # TODO: better support
           prep_ty = Type::Instance.new(genv, prep_mod, type_params)
         end
         if prep_ty.mod == f_mod
@@ -58,7 +58,7 @@ def self.typecheck_for_included_modules(genv, changes, a_ty, f_mod, f_args, subs
         if inc_decl.is_a?(AST::SigIncludeNode) && inc_mod.type_params
           inc_ty = genv.get_instance_type(inc_mod, inc_decl.args, changes, {}, a_ty)
         else
-          type_params = inc_mod.type_params.map {|ty_param| Source.new() } # TODO: better support
+          type_params = inc_mod.type_params.map {|(_name, _default_ty)| Source.new() } # TODO: better support
           inc_ty = Type::Instance.new(genv, inc_mod, type_params)
         end
         if inc_ty.mod == f_mod
diff --git a/lib/typeprof/core/env.rb b/lib/typeprof/core/env.rb
index 13a4a8e5..4facb095 100644
--- a/lib/typeprof/core/env.rb
+++ b/lib/typeprof/core/env.rb
@@ -113,16 +113,20 @@ def get_superclass(singleton, mod)
     def get_instance_type(mod, type_args, changes, base_ty_env, base_ty)
       ty_env = base_ty_env.dup
       if base_ty.is_a?(Type::Instance)
-        base_ty.mod.type_params.zip(base_ty.args) do |param, arg|
-          ty_env[param] = arg || Source.new
+        base_ty.mod.type_params.zip(base_ty.args) do |(param, default_ty), arg|
+          ty_env[param] = arg || (default_ty ? default_ty.covariant_vertex(self, changes, ty_env) : Source.new)
         end
       elsif base_ty.is_a?(Type::Singleton)
-        base_ty.mod.type_params&.each do |param|
-          ty_env[param] = Source.new
+        base_ty.mod.type_params&.each do |(param, default_ty)|
+          ty_env[param] = default_ty ? default_ty.covariant_vertex(self, changes, ty_env) : Source.new
         end
       end
-      args = mod.type_params.zip(type_args).map do |param, arg|
-        arg && changes ? arg.covariant_vertex(self, changes, ty_env) : Source.new
+      args = mod.type_params.zip(type_args).map do |(param, default_ty), arg|
+        if changes
+          (arg || default_ty)&.covariant_vertex(self, changes, ty_env) || Source.new
+        else
+          Source.new
+        end
       end
       Type::Instance.new(self, mod, args)
     end
@@ -380,7 +384,7 @@ def get_self(genv)
       case @scope_level
       when :instance
         mod = genv.resolve_cpath(@cpath || [])
-        type_params = mod.type_params.map {|ty_param| Source.new() } # TODO: better support
+        type_params = mod.type_params.map {|(_name, _default_ty)| Source.new() } # TODO: better support
         ty = Type::Instance.new(genv, mod, type_params)
         Source.new(ty)
       when :class
diff --git a/lib/typeprof/core/env/module_entity.rb b/lib/typeprof/core/env/module_entity.rb
index 005a83ce..9ca3d39c 100644
--- a/lib/typeprof/core/env/module_entity.rb
+++ b/lib/typeprof/core/env/module_entity.rb
@@ -25,7 +25,7 @@ def initialize(cpath, outer_module = self)
 
       # class Foo[X, Y, Z] < Bar[A, B, C]
       @superclass_type_args = nil # A, B, C
-      @type_params = [] # X, Y, Z
+      @type_params = {} # X, Y, Z
 
       @consts = {}
       @methods = { true => {}, false => {} }
@@ -115,9 +115,10 @@ def add_module_decl(genv, decl)
       @module_decls << decl
 
       if @type_params
-        update_type_params if @type_params != decl.params
+        update_type_params
       else
-        @type_params = decl.params
+        @type_params = {}
+        decl.params.zip(decl.params_default_types) {|name, default_type| @type_params[name] = default_type }
       end
 
       if decl.is_a?(AST::SigClassNode) && !@superclass_type_args
@@ -133,7 +134,7 @@ def remove_module_decl(genv, decl)
       @outer_module.get_const(get_cname).remove_decl(decl)
       @module_decls.delete(decl) || raise
 
-      update_type_params if @type_params == decl.params
+      update_type_params
       if decl.is_a?(AST::SigClassNode) && @superclass_type_args == decl.superclass_args
         @superclass_type_args = nil
         @module_decls.each do |decl|
@@ -152,13 +153,12 @@ def update_type_params
       @module_decls.each do |decl|
         params = decl.params
         next unless params
-        if @type_params
-          @type_params = params if (@type_params <=> params) > 0
-        else
-          @type_params = params
+        if !@type_params || @type_params.size < params.size
+          @type_params = {}
+          params.zip(decl.params_default_types) {|name, default_type| @type_params[name] = default_type }
         end
       end
-      @type_params ||= []
+      @type_params ||= {}
       # TODO: report an error if there are multiple inconsistent declarations
     end
 
diff --git a/lib/typeprof/core/graph/box.rb b/lib/typeprof/core/graph/box.rb
index 103bb0d9..9c62aeb6 100644
--- a/lib/typeprof/core/graph/box.rb
+++ b/lib/typeprof/core/graph/box.rb
@@ -87,7 +87,7 @@ def run0(genv, changes)
         mod = genv.resolve_cpath(@node.cpath)
         if mod.type_params && !mod.type_params.empty?
           # Create a substitution map where each type parameter maps to a type variable vertex
-          subst = mod.type_params.to_h do |param|
+          subst = mod.type_params.to_h do |param, _default_ty|
             type_var_vtx = Vertex.new(@node)
             [param, type_var_vtx]
           end
@@ -185,8 +185,8 @@ def match_arguments?(genv, changes, param_map, a_args, method_type)
     def resolve_overload(changes, genv, method_type, node, param_map, a_args, ret, force)
       param_map0 = param_map.dup
       if method_type.type_params
-        method_type.type_params.zip(yield(method_type)) do |var, vtx|
-          param_map0[var] = vtx
+        method_type.type_params.zip(yield(method_type)) do |(var, _default_ty), vtx|
+          param_map0[var] = vtx # TODO: default_ty?
         end
       end
 
@@ -443,17 +443,17 @@ def run0(genv, changes)
         ty = Type::Singleton.new(genv, mod)
         param_map0 = Type.default_param_map(genv, ty)
       else
-        type_params = mod.type_params.map {|ty_param| Source.new() } # TODO: better support
+        type_params = mod.type_params.map {|(_name, _default_ty)| Source.new() } # TODO: better support
         ty = Type::Instance.new(genv, mod, type_params)
         param_map0 = Type.default_param_map(genv, ty)
         if ty.is_a?(Type::Instance)
-          ty.mod.type_params.zip(ty.args) do |param, arg|
-            param_map0[param] = arg
+          ty.mod.type_params.zip(ty.args) do |(name, _default_ty), arg|
+            param_map0[name] = arg
           end
         end
       end
-      method_type.type_params.each do |param|
-        param_map0[param] = Source.new()
+      method_type.type_params.each do |name, _default_ty|
+        param_map0[name] = Source.new()
       end
 
       positional_args = []
@@ -739,12 +739,12 @@ def run0(genv, changes)
             # TODO: add_depended_method_entity for types used to resolve overloads
             ty_env = Type.default_param_map(genv, orig_ty)
             if ty.is_a?(Type::Instance)
-              ty.mod.type_params.zip(ty.args) do |param, arg|
-                ty_env[param] = arg
+              ty.mod.type_params.zip(ty.args) do |(param, default_ty), arg|
+                ty_env[param] = arg || (default_ty ? default_ty.covariant_vertex(genv, changes, ty_env) : Source.new)
               end
             end
             mdecl.resolve_overloads(changes, genv, @node, ty_env, @a_args, @ret) do |method_type|
-              @generics[method_type] ||= method_type.type_params.map {|var| Vertex.new(@node) }
+              @generics[method_type] ||= method_type.type_params.map { Vertex.new(@node) }
             end
           end
         elsif !me.defs.empty?
@@ -846,7 +846,7 @@ def resolve_prepended_modules(genv, changes, base_ty_env, ty, mid, &blk)
         if prep_decl.is_a?(AST::SigPrependNode) && prep_mod.type_params
           prep_ty = genv.get_instance_type(prep_mod, prep_decl.args, changes, base_ty_env, ty)
         else
-          type_params = prep_mod.type_params.map {|ty_param| Source.new() } # TODO: better support
+          type_params = prep_mod.type_params.map { Source.new() } # TODO: better support
           prep_ty = Type::Instance.new(genv, prep_mod, type_params)
         end
 
@@ -901,7 +901,7 @@ def resolve_included_modules(genv, changes, base_ty_env, ty, mid, &blk)
         if inc_decl.is_a?(AST::SigIncludeNode) && inc_mod.type_params
           inc_ty = genv.get_instance_type(inc_mod, inc_decl.args, changes, base_ty_env, ty)
         else
-          type_params = inc_mod.type_params.map {|ty_param| Source.new() } # TODO: better support
+          type_params = inc_mod.type_params.map { Source.new() } # TODO: better support
           inc_ty = Type::Instance.new(genv, inc_mod, type_params)
         end
 
diff --git a/lib/typeprof/core/type.rb b/lib/typeprof/core/type.rb
index 960840d3..cda5377b 100644
--- a/lib/typeprof/core/type.rb
+++ b/lib/typeprof/core/type.rb
@@ -60,7 +60,7 @@ def show
 
       def get_instance_type(genv)
         params = @mod.type_params
-        Instance.new(genv, @mod, params ? params.map { Source.new } : [])
+        Instance.new(genv, @mod, params ? params.map { Source.new } : []) # TODO: respect param_default_types
       end
     end
 
diff --git a/scenario/rbs/param-default-type.rb b/scenario/rbs/param-default-type.rb
new file mode 100644
index 00000000..d2af2013
--- /dev/null
+++ b/scenario/rbs/param-default-type.rb
@@ -0,0 +1,27 @@
+## update: test.rbs
+class Foo[X, Y = Integer]
+  def get_x: -> X
+  def get_y: -> Y
+end
+
+class Object
+  def create_foo_str_int: -> Foo[String]
+  def create_foo_str_str: -> Foo[String, String]
+end
+
+## update: test.rb
+def check1
+  x = create_foo_str_int
+  [x.get_x, x.get_y]
+end
+
+def check2
+  x = create_foo_str_str
+  [x.get_x, x.get_y]
+end
+
+## assert
+class Object
+  def check1: -> [String, Integer]
+  def check2: -> [String, String]
+end