diff --git a/ChangeLog b/ChangeLog
index 1aa6a398e3..20dab87235 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,14 @@
+Tue May 18 00:05:00 2016  Kenta Murata  <mrkn@mrkn.jp>
+
+	* enum.c (enum_sum): Optimize for a range from int to int.
+
+	* test/ruby/test_enum.rb (test_range_sum): Move from test_range.rb,
+	  and add assertions for some conditions.
+
+	* test/ruby/test_enum.rb (test_hash_sum): Move from test_hash.rb.
+
+	* test/ruby/test_hash.rb, test/ruby/test_range.rb: Remove test_sum.
+
 Tue May 17 23:08:00 2016  Kenta Murata  <mrkn@mrkn.jp>
 
 	* enum.c (enum_sum): [DOC] Write documentation.
diff --git a/enum.c b/enum.c
index 56d304c7a9..54e1364de7 100644
--- a/enum.c
+++ b/enum.c
@@ -3699,6 +3699,8 @@ static VALUE
 enum_sum(int argc, VALUE* argv, VALUE obj)
 {
     struct enum_sum_memo memo;
+    VALUE beg, end;
+    int excl;
 
     if (rb_scan_args(argc, argv, "01", &memo.v) == 0)
         memo.v = LONG2FIX(0);
@@ -3713,6 +3715,29 @@ enum_sum(int argc, VALUE* argv, VALUE obj)
         memo.c = 0.0;
     }
 
+    if (RTEST(rb_range_values(obj, &beg, &end, &excl))) {
+        if (!memo.block_given && !memo.float_value &&
+                (FIXNUM_P(beg) || RB_TYPE_P(beg, T_BIGNUM)) &&
+                (FIXNUM_P(end) || RB_TYPE_P(end, T_BIGNUM))) {
+            if (excl) {
+                if (FIXNUM_P(end))
+                    end = LONG2FIX(FIX2LONG(end) - 1);
+                else
+                    end = rb_big_minus(end, LONG2FIX(1));
+            }
+            if (rb_int_ge(end, beg)) {
+                VALUE a;
+                a = rb_int_plus(rb_int_minus(end, beg), LONG2FIX(1));
+                a = rb_int_mul(a, rb_int_plus(end, beg));
+                a = rb_int_idiv(a, LONG2FIX(2));
+                return rb_int_plus(memo.v, a);
+            }
+            else {
+                return memo.v;
+            }
+        }
+    }
+
     rb_block_call(obj, id_each, 0, 0, enum_sum_iter_i, (VALUE)&memo);
 
     if (memo.float_value) {
diff --git a/internal.h b/internal.h
index 78ae77d806..2816737390 100644
--- a/internal.h
+++ b/internal.h
@@ -1097,6 +1097,7 @@ VALUE rb_int_round(VALUE num, int ndigits);
 VALUE rb_int2str(VALUE num, int base);
 VALUE rb_dbl_hash(double d);
 VALUE rb_fix_plus(VALUE x, VALUE y);
+VALUE rb_int_ge(VALUE x, VALUE y);
 
 #if USE_FLONUM
 #define RUBY_BIT_ROTL(v, n) (((v) << (n)) | ((v) >> ((sizeof(v) * 8) - n)))
diff --git a/numeric.c b/numeric.c
index 0ebaa95ecb..c61f5d0982 100644
--- a/numeric.c
+++ b/numeric.c
@@ -3908,8 +3908,8 @@ fix_ge(VALUE x, VALUE y)
     }
 }
 
-static VALUE
-int_ge(VALUE x, VALUE y)
+VALUE
+rb_int_ge(VALUE x, VALUE y)
 {
     if (FIXNUM_P(x)) {
 	return fix_ge(x, y);
@@ -4950,7 +4950,7 @@ Init_Numeric(void)
     rb_define_method(rb_cInteger, "===", int_equal, 1);
     rb_define_method(rb_cInteger, "==", int_equal, 1);
     rb_define_method(rb_cInteger, ">", int_gt, 1);
-    rb_define_method(rb_cInteger, ">=", int_ge, 1);
+    rb_define_method(rb_cInteger, ">=", rb_int_ge, 1);
     rb_define_method(rb_cInteger, "<", int_lt, 1);
     rb_define_method(rb_cInteger, "<=", int_le, 1);
 
diff --git a/test/ruby/test_enum.rb b/test/ruby/test_enum.rb
index ed464a285e..df75a80c50 100644
--- a/test/ruby/test_enum.rb
+++ b/test/ruby/test_enum.rb
@@ -905,4 +905,20 @@ class TestEnumerable < Test::Unit::TestCase
       assert_equal(6, [1r, 2, 3r].each.sum)
     EOS
   end
+
+  def test_hash_sum
+    histogram = { 1 => 6, 2 => 4, 3 => 3, 4 => 7, 5 => 5, 6 => 4 }
+    assert_equal(100, histogram.sum {|v, n| v * n })
+  end
+
+  def test_range_sum
+    assert_int_equal(55, (1..10).sum)
+    assert_float_equal(55.0, (1..10).sum(0.0))
+    assert_int_equal(90, (5..10).sum {|v| v * 2 })
+    assert_float_equal(90.0, (5..10).sum(0.0) {|v| v * 2 })
+    assert_int_equal(0, (2..0).sum)
+    assert_int_equal(5, (2..0).sum(5))
+    assert_int_equal(2, (2..2).sum)
+    assert_int_equal(42, (2...2).sum(42))
+  end
 end
diff --git a/test/ruby/test_hash.rb b/test/ruby/test_hash.rb
index 03a712d85b..6a8e7f0423 100644
--- a/test/ruby/test_hash.rb
+++ b/test/ruby/test_hash.rb
@@ -1415,11 +1415,6 @@ class TestHash < Test::Unit::TestCase
     assert_equal([10, 20, 30], [1, 2, 3].map(&h))
   end
 
-  def test_sum
-    histogram = { 1 => 6, 2 => 4, 3 => 3, 4 => 7, 5 => 5, 6 => 4 }
-    assert_equal(100, histogram.sum {|v, n| v * n })
-  end
-
   class TestSubHash < TestHash
     class SubHash < Hash
       def reject(*)
diff --git a/test/ruby/test_range.rb b/test/ruby/test_range.rb
index 431ed0dc30..2bff105936 100644
--- a/test/ruby/test_range.rb
+++ b/test/ruby/test_range.rb
@@ -631,9 +631,4 @@ class TestRange < Test::Unit::TestCase
     end
     (a.."c").each {|x, &b| assert_nil(b)}
   end
-
-  def test_sum
-    assert_equal(55, (1..10).sum)
-    assert_equal(110, (1..10).sum {|v| v * 2 })
-  end
 end