diff --git a/ChangeLog b/ChangeLog
index e684abc083..c9463fea4b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Mon Sep 19 17:05:22 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* numeric.c (Init_Numeric), bignum.c (Init_Bignum): deprecate
+	  Fixnum and Bignum.  this may be reverted after previews.
+	  [Feature #12739]
+
 Mon Sep 19 10:36:53 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* enumerator.c (lazy_init_yielder): directly call stored functions.
diff --git a/bignum.c b/bignum.c
index 741fa2d21e..ca06c54b3e 100644
--- a/bignum.c
+++ b/bignum.c
@@ -6781,6 +6781,7 @@ Init_Bignum(void)
     rb_cBignum = rb_cInteger;
 #endif
     rb_define_const(rb_cObject, "Bignum", rb_cInteger);
+    rb_deprecate_constant(rb_cObject, "Bignum");
 
     rb_define_method(rb_cInteger, "coerce", rb_int_coerce, 1);
 
diff --git a/numeric.c b/numeric.c
index ffa576149e..5ef02336db 100644
--- a/numeric.c
+++ b/numeric.c
@@ -5182,6 +5182,7 @@ Init_Numeric(void)
     rb_cFixnum = rb_cInteger;
 #endif
     rb_define_const(rb_cObject, "Fixnum", rb_cInteger);
+    rb_deprecate_constant(rb_cObject, "Fixnum");
 
     rb_cFloat  = rb_define_class("Float", rb_cNumeric);