login as root on Ubuntu

First of all. you cannot login as root after you installed Ubuntu.

But you could do: $sudo su, which will give you root permission. but you are not root user.

you also could login as root by setting the root password
$sudo passwd
you could enter your root password. Then you could login as root.

Remember, there is a difference between
$su
and
$sudo su

$su has more privilege than $sudo su