Linux operating systems like Ubuntu do not allow logging in as root users due to security reasons. As an alternative users have to perform root actions (such as installing software) by using sudo commands. While it can be quite risky to login as root user, one may be willing to perform administrative functions in the GUI mode. In case you would like to login to Ubuntu using GUI, here is a small tip which will allow you to do just that.
Disclaimer: Logging in as root user can cause damage to operating system in case of the slightest of error at the end users part. Use this method at your own risk.
Open a Terminal and enter the following command
sudo passwd root
You will be prompted to enter system password followed by the UNIX password. For the New UNIX password, enter the root password.
Now simply log out of your current session and login with the username “root” and the password which you assigned in the above step. Make sure you do not perform any action which may lead to serious system havoc. It is not advised to experiment around with your OS, when logged in as root user.