[CentOS] sysctl setting reset

Hello, 

I have found the sysctl setting to net.ipv4.netfilter.ip_conntrack_max value reset everyday, may I know is there a way to set it permanently?

I have set it to 150000, but it reset to 65535 in few hours.

The running Centos is version 4 and kernel version is 2.6.9-55.0.12.ELsmp.

Do I need to reboot to have it set permanently? It seems that I need to run sysctl -p to re-read the /etc/sysctl.cnf.

Thanks.

- Steve