When the first time you installed Ubuntu, there is no password for root account. You have to set it first for a security reason.

You must type “sudo passwd”

[email protected]:~$ sudo passwd
[sudo] password for hostmaster:
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

To login as root, type “su”

[email protected]:~$ su
[email protected]:/home/hostmaster#

The “#” indicate that you are login as root.