Can i turn on, logout, login one ubuntu pc from another ubuntu pc through remote desktop. I have also try remote desktop but i can only when pc is turn on.
Powered off → unreachable.
You'll only be able to connect to your PC whenever it's on, because if it's off, then there's nothing running on the machine to contact to and there's no network active. A device in a powered off state is really off, you see. Similarly - your can't receive phone calls on your mobile phone without it having turned on. Some software can't do magic here.
However, I know of two techniques exist to manage the power without the machine being turned on, although it needs the wall socket power to be connected of course.
After the machine has been powered on with one of the techniques above, you'll have to make sure your remote desktop server (e.g. VNC server, NX server, etc.) is started and the network is automatically connected properly, so you can connect to it.