diff --git a/ChangeLog b/ChangeLog
index fc9d51295d..5f2e232c6e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Sat Mar 20 13:26:09 2010  Tanaka Akira  <akr@fsij.org>
+
+	* array.c (rb_ary_uniq_bang): the array is already unique if the
+	  length is zero or one.
+	  (rb_ary_uniq): ditto.
+
 Sat Mar 20 12:30:54 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib: fixed typo.  a patch by Sho Hashimoto in [ruby-dev:40716].
diff --git a/array.c b/array.c
index 138b3cfdb9..744d1e0491 100644
--- a/array.c
+++ b/array.c
@@ -3367,6 +3367,8 @@ rb_ary_uniq_bang(VALUE ary)
     long i, j;
 
     rb_ary_modify_check(ary);
+    if (RARRAY_LEN(ary) <= 1)
+        return Qnil;
     if (rb_block_given_p()) {
 	hash = ary_make_hash_by(ary);
 	if (RARRAY_LEN(ary) == (i = RHASH_SIZE(hash))) {
@@ -3412,6 +3414,8 @@ rb_ary_uniq(VALUE ary)
     VALUE hash, uniq, v;
     long i;
 
+    if (RARRAY_LEN(ary) <= 1)
+        return rb_ary_dup(ary);
     if (rb_block_given_p()) {
 	hash = ary_make_hash_by(ary);
 	uniq = ary_new(rb_obj_class(ary), RHASH_SIZE(hash));
diff --git a/test/ruby/test_array.rb b/test/ruby/test_array.rb
index 98acfe9d29..69d3f1d75c 100644
--- a/test/ruby/test_array.rb
+++ b/test/ruby/test_array.rb
@@ -1308,6 +1308,30 @@ class TestArray < Test::Unit::TestCase
   end
 
   def test_uniq
+    a = []
+    b = a.uniq
+    assert_equal([], a)
+    assert_equal([], b)
+    assert_not_same(a, b)
+
+    a = [1]
+    b = a.uniq
+    assert_equal([1], a)
+    assert_equal([1], b)
+    assert_not_same(a, b)
+
+    a = [1,1]
+    b = a.uniq
+    assert_equal([1,1], a)
+    assert_equal([1], b)
+    assert_not_same(a, b)
+
+    a = [1,2]
+    b = a.uniq
+    assert_equal([1,2], a)
+    assert_equal([1,2], b)
+    assert_not_same(a, b)
+
     a = @cls[ 1, 2, 3, 2, 1, 2, 3, 4, nil ]
     b = a.dup
     assert_equal(@cls[1, 2, 3, 4, nil], a.uniq)
@@ -1322,6 +1346,25 @@ class TestArray < Test::Unit::TestCase
   end
 
   def test_uniq!
+    a = []
+    b = a.uniq!
+    assert_equal(nil, b)
+
+    a = [1]
+    b = a.uniq!
+    assert_equal(nil, b)
+
+    a = [1,1]
+    b = a.uniq!
+    assert_equal([1], a)
+    assert_equal([1], b)
+    assert_same(a, b)
+
+    a = [1,2]
+    b = a.uniq!
+    assert_equal([1,2], a)
+    assert_equal(nil, b)
+
     a = @cls[ 1, 2, 3, 2, 1, 2, 3, 4, nil ]
     assert_equal(@cls[1, 2, 3, 4, nil], a.uniq!)
     assert_equal(@cls[1, 2, 3, 4, nil], a)