if i turn off cold water valve at hot water heater will the pressure in hot water line be empty?
Short answer: Yes
It depends on where the water heater is, relative to the pipes you want to work on. With a typical water heater in the basement, there will be very little pressure. If the water heater is on one level and you want to work on pipes a floor below then you may have to drain the entire tank. In any case, open up a couple of hot faucets and let them run until dry and you're all set.
Keep in mind that if you have mixing valves then you might have a problem of cold crossing to the hot side. If so, you may need to turn off the main supply instead of just the hot water, even though you're only working on the hot pipes.