iptables can not start

Today,I want to start iptables on my CentOS box for security reason
I type
service iptables start
but there is no output
then I type
service iptables status
it shows
Firewall is stopped.
After some googles,I solve it
type this command
system-config-securitylevel
set Security Level to be Enabled
hit ok
at last
service iptables start