Full screen for ubuntu on VM VirtualBox

I have VM VirtualBox on Win 7, and Ubuntu on VM VirtualBox. By default, the ubuntu could not be displayed as fullscreen. But there is way to make it seem as the ubuntu os is directly installed on your PC hardisk.

1. Installing Guest Additions for full screen

   [Device]--->[Install Guest Addtions]

2. After some configure by the machine, you can open the Guest Additions folder which should be on your Desktop in Ubuntu with name like this: VBOXADDITIONS_#

3. Open terminal, and cd into this folder VBOXADDITIONS_# by:

cd  /media

cd  VBOXADDITIONS_# 

sudo sh VboxLinuxAdditions.run

4. Restart the ubuntu.
you can see the ubuntu is enlarged filling up the whole VirtualBox. However the VirtualBox window frame still exists, you can implement the real full screen by HOST+F,  which would make  the VirtualBox window frame  disappear. Another  HOST+F will bring the window frame back again. By default in Ubuntu, the HOST key is the Right Ctrl