In this paper a Twin Platform Bed computer program is employed to evaluate the performance of a gas turbine power plant at a different ambient temperature.This is very important especially in Iraq, because of the wide range of ambient temperature variation throughout the year.The results show that the thermal efficiency of the plant decreases as the ambient air temperature increases, where the drop in efficiency is significant at higher temperatures, especially in summer season.So, it is very important to use a cooling system to control the ambient REGULAR SAUSAGE temperature for best performance.The study is done on a single shaft, simple cycle gas turbine power plant with maximum output power of 20 MW.