Abstract: Combined cycle gas turbines are becoming popular to product power at higher efficiencies with reduced pollutants and greenhouse gas emissions. In the present research, the effect of steam injection inside gas turbine combustion chamber and isentropic component efficiencies on the performance of combined cycle gas turbine is investigated. For a particular combined cycle gas turbine configuration, the effect of steam injection in the gas turbine combustor on the performance of gas turbine cycle, steam turbine cycle and fuel consumption is investigated based on 1st law of thermodynamics. Also for a particular combined cycle configuration, the effect of component efficiencies of gas turbine, compressors and the steam turbine on the combined cycle efficiency and the net power output is also conducted. The effect of operation conditions specially the pressure ratio for the gas turbine cycle and the turbine inlet temperature on the performance of combined cycle and gas turbine cycle are investigate and discuss. The result shown the power output increases about 21% with increases the pressure ratio from 5-35 and increases the fraction of steam injection from 0-10%.