diff --git a/ChangeLog b/ChangeLog
index 56b9a48872..09c310310c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Wed Apr 15 20:24:49 2009  Yusuke Endoh  <mame@tsg.ne.jp>
+
+	* array.c (rb_ary_flatten): flatten(0) works as Array#dup.
+	  [ruby-core:23168]
+
+	* test/ruby/test_array.rb: add a test for above.
+
 Wed Apr 15 11:53:35 2009  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* dir.c (fnmatch_helper): use rb_enc_precise_mbclen and
diff --git a/array.c b/array.c
index b5f1bcf93b..6da6ef9639 100644
--- a/array.c
+++ b/array.c
@@ -648,6 +648,12 @@ ary_make_partial(VALUE ary, VALUE klass, long offset, long len)
     }
 }
 
+static VALUE
+ary_make_shared_copy(VALUE ary)
+{
+    return ary_make_partial(ary, rb_obj_class(ary), 0, RARRAY_LEN(ary));
+}
+
 enum ary_take_pos_flags
 {
     ARY_TAKE_FIRST = 0,
@@ -3360,7 +3366,7 @@ rb_ary_flatten(int argc, VALUE *argv, VALUE ary)
 
     rb_scan_args(argc, argv, "01", &lv);
     if (!NIL_P(lv)) level = NUM2INT(lv);
-    if (level == 0) return ary;
+    if (level == 0) return ary_make_shared_copy(ary);
 
     result = flatten(ary, level, &mod);
     OBJ_INFECT(result, ary);
diff --git a/test/ruby/test_array.rb b/test/ruby/test_array.rb
index 92c309241f..9b3dfdf45f 100644
--- a/test/ruby/test_array.rb
+++ b/test/ruby/test_array.rb
@@ -719,6 +719,11 @@ class TestArray < Test::Unit::TestCase
     a7 = a6.flatten
     assert_equal(true, a7.tainted?)
     assert_equal(true, a7.untrusted?)
+
+    a8 = @cls[[1, 2], 3]
+    a9 = a8.flatten(0)
+    assert_equal(a8, a9)
+    assert_not_same(a8, a9)
   end
 
   def test_flatten!