The coolant in my car keeps evaporating for unknown reasons. I didnt notice any leaks coming from the car, but after about 200 miles the car doesnt have any water. I believe the head gasket is not bad because theres no water in the oil and i took the car to the shop and the compresions test is good. The shop said it might be termostate,raditor need flush, and raditor cap. I replace the raditor, changed the cap and termostate about 10 months ago. There are dry water stans around the cap and resevoir area. Whats the the possiblity that its still can be a gasket problem?

You took it to a shop for a compression test? I hope they didnt charge you more than $10... Anyways, try burping the cooling system, you'd be surprised how often this helps if not fixes problems. Secondly, does this happen at idle only or all the time? If its just at idle make yourself a jumper and confirm your fan is turning on.

I purchased 3 radiator caps in a year because of a similar problem. I wasn't mixing enough antifreeze to water and it would steam out, of the cap, boil over etc. and it would harden the rubber seal cracking it pretty quickly. That was in the early days. I've since learned. Other things that can cause you to loose coolant/over heat and not see leaking which I have also experienced over the last ten years are:

poor sealing Tstat housing gasket, leaking against the head and buring away slowly

poor sealing at any of the hose locations where they mate to hard pipe not to mention small holes that can develop for various reasons also leaking trace amounts and burning away

heater core leak crack, leaking into car (kinda noticable by wet carpet and smell of coolant)

small holes in the radiator which will also burn away as they leak.

Those are just off the top of my head.