diff --git a/ChangeLog b/ChangeLog
index 0cb6e09372..c4adad6dea 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Mon Dec  9 16:13:31 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* gc.c (wmap_size): add ObjectSpace::WeakMap#size and #length.
+
 Mon Dec  9 15:26:17 2013  Shugo Maeda  <shugo@ruby-lang.org>
 
 	* test/test_curses.rb: removed.
diff --git a/gc.c b/gc.c
index 7f1a4983d9..2083cc1027 100644
--- a/gc.c
+++ b/gc.c
@@ -6470,6 +6470,20 @@ wmap_has_key(VALUE self, VALUE key)
     return NIL_P(wmap_aref(self, key)) ? Qfalse : Qtrue;
 }
 
+static VALUE
+wmap_size(VALUE self)
+{
+    struct weakmap *w;
+    st_index_t n;
+
+    TypedData_Get_Struct(self, struct weakmap, &weakmap_type, w);
+    n = w->wmap2obj->num_entries;
+#if SIZEOF_ST_INDEX_T <= SIZEOF_LONG
+    return ULONG2NUM(n);
+#else
+    return ULL2NUM(n);
+#endif
+}
 
 /*
   ------------------------------ GC profiler ------------------------------
@@ -7304,6 +7318,8 @@ Init_GC(void)
 	rb_define_method(rb_cWeakMap, "each_value", wmap_each_value, 0);
 	rb_define_method(rb_cWeakMap, "keys", wmap_keys, 0);
 	rb_define_method(rb_cWeakMap, "values", wmap_values, 0);
+	rb_define_method(rb_cWeakMap, "size", wmap_size, 0);
+	rb_define_method(rb_cWeakMap, "length", wmap_size, 0);
 	rb_define_private_method(rb_cWeakMap, "finalize", wmap_finalize, 1);
 	rb_include_module(rb_cWeakMap, rb_mEnumerable);
     }
diff --git a/test/ruby/test_weakmap.rb b/test/ruby/test_weakmap.rb
index 383b5e1f31..c0d1747d2a 100644
--- a/test/ruby/test_weakmap.rb
+++ b/test/ruby/test_weakmap.rb
@@ -93,4 +93,18 @@ class TestWeakMap < Test::Unit::TestCase
     end
     assert_equal(2, n)
   end
+
+  def test_size
+    m = __callee__[/test_(.*)/, 1]
+    assert_equal(0, @wm.__send__(m))
+    x1 = "foo"
+    k1 = Object.new
+    @wm[k1] = x1
+    assert_equal(1, @wm.__send__(m))
+    x2 = "bar"
+    k2 = Object.new
+    @wm[k2] = x2
+    assert_equal(2, @wm.__send__(m))
+  end
+  alias test_length test_size
 end