You may found that in ubuntu 12.04 we can not login as root like earlier version of ubuntu.That’s easy,just follow below steps to enable login as root in ubuntu 12.04
1.Open a Terminal by pressing Ctl+Alt+t or search Terminal in Dash
2.Run below commands to set root a password and update /etc/lightdm/lightdm.conf to enable login as root
sudo passwd root sudo sh -c 'echo "greeter-show-manual-login=true" >> /etc/lightdm/lightdm.conf'
3.Reboot your computer,you can see there is a login button in login screen,you can click this button then login as root
4.Optional,if you want to disable login as root,just sudo and use your favorite text editor to remove the last line of /etc/lightdm/lightdm.conf then reboot your computer