reference: http://www.linuxtopia.org/online_books/centos_linux_guides/centos_linux_step_by_step_guide/s1-q-and-a-root-passwd.html

Forgotten root Password

1. At the boot loader menu, use the arrow keys to highlight the installation you want to edit and type [A] to enter into append mode.

2. You are presented with a prompt that looks similar to the following:

   grub append> ro root=LABEL=/

3. Press the Spacebar once to add a blank space, then add the word single to tell GRUB to boot into single-user Linux mode. The result should look like the following:

   ro root=LABEL=/ single

4. Press [Enter] and GRUB will boot single-user Linux mode. After it finishes loading, you will be presented with a shell prompt similar to the following:

   sh-2.05b#

5. You can now change the root password by typing

   passwd root

   You will be asked to re-type the password for verification. Once you are finished, the password will be changed. You can then reboot by typing reboot at the prompt; then you can log in to root as you normally would.