Whenever you can't remember Root password, you can read this tutorial and do step by step to Reset it ...

You can log in with single-user mode and create a new root password.
Reboot your computer. When GRUB is presenting the menu list, follow those instructions:

• use the arrows to select the boot entry you want to modify
• press e to edit the entry
• use the arrows to go to kernel line
• press e to edit this entry
• at the end of the line add the word single (there should be a space between like  / single) 
• press ENTER to go back to the parent menu
• press b to boot this kernel

As root, changing password does not ask for your old password. Now, you can change root's password by typing:

bash# passwd root

You'll be asked to re-type the password for verification. Once you're finished, the password will be changed and you can reboot by typing shutdown -r now at the prompt; then you can log in to root as before