diff --git a/ChangeLog b/ChangeLog
index 5f0688b88d..def8de8d0f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed May 12 15:04:57 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* ext/bigdecimal/lib/bigdecimal/math.rb (BigMath#log): improved
+	  precision and performance. [ruby-dev:41295]
+
 Wed May 12 11:39:10 2010  NAKAMURA Usaku  <usa@ruby-lang.org>
 
 	* ext/socket/extconf.rb: test all IPPROTO_* constants for recent Win32
diff --git a/ext/bigdecimal/lib/bigdecimal/math.rb b/ext/bigdecimal/lib/bigdecimal/math.rb
index 07efcbe099..41fc69f66c 100644
--- a/ext/bigdecimal/lib/bigdecimal/math.rb
+++ b/ext/bigdecimal/lib/bigdecimal/math.rb
@@ -190,8 +190,10 @@ module BigMath
     one = BigDecimal("1")
     two = BigDecimal("2")
     n  = prec + BigDecimal.double_fig
-    if (expo = x.exponent) < 0
+    if (expo = x.exponent) < 0 || expo >= 3
       x = x.mult(BigDecimal("1E#{-expo}"), n)
+    else
+      expo = nil
     end
     x  = (x - one).div(x + one,n)
     x2 = x.mult(x,n)
@@ -206,7 +208,7 @@ module BigMath
       y += d
     end
     y *= two
-    if expo < 0
+    if expo
       y += log(BigDecimal("10"),prec) * BigDecimal(expo.to_s)
     end
     y