Just follow the steps below

#grub

grub>md5crypt

Password: ******

Encrypted: $1$jxcdN0$hVHViq1aiPf8FziuGJGZp0

/*Copy down encrypted password: */

$1$jxcdN0$hVHViq1aiPf8FziuGJGZp0

/*quit grub now by entering following command*/

grub> quit

#vim /etc/grub.conf

Insert encrypted password in between "splashimage..." and "title...": 

Save edited fileand quit

:wq

#reboot

---done------

Now u cant edit the grub menu with to enter and modify u need to giive the grub password by pressing P