Actually, I tried many ways to solve this problem. It seems that something wrong with my lightdm software so, at last, I use gdm software to replace lightdm to kill this problem. Just install gdm in Ubuntu Precise as follows.
$ sudo apt-get install gdm
Note: It would let you to select gdm to replace lightdm
LightDM is an X display manager that aims to be lightweight, fast, extensible and multi-desktop. It uses various front-ends to draw login interfaces, so-called Greeters.
GDM: It stands for Gnome Display Manager.