diff --git a/build.sh b/build.sh
index 37fe0ea..06530a7 100755
--- a/build.sh
+++ b/build.sh
@@ -39,6 +39,11 @@
   echo ""
 }
 
+if test "$(uname)" = "Linux"; then
+  declare cpu_num=$(cat /proc/cpuinfo | grep "processor" | wc -l)
+elif test "$(uname)" = "Darwin" ; then
+  declare cpu_num=$(sysctl -n machdep.cpu.thread_count)
+fi
 declare need_build_jsoncpp=1
 declare need_build_libevent=1
 declare need_build_boost=1
@@ -46,7 +51,6 @@
 declare enable_lsan=0
 declare verbose=1
 declare codecov=0
-declare cpu_num=4
 declare test=0
 
 pasres_arguments() {