ubuntu has no password for root user as default. If you want to reset root password, you need to use sudo passwd command in shell. To Open shell, click Applications » Accessories » Terminal. In shell, type sudo passwd. It will ask you password. Type your login password.
myochauhtun@myochauhtun-desktop:~$ sudo passwd
And then it will ask you to reset password for root. Type it twice
Enter new UNIX password:
Retype new UNIX password:
After reset root password. It will show this message in Shell.
passwd: password updated successfully
Now you can login as root by typing su command
regards : How to change root password in Ubuntu