The power output and efficiency of gas turbine is a strong function of the ambient air temperature. In hot climatic condition the mass flow rate of air decreases with increasing ambient air temperature for the same volumetric flow rate, which results in reduction in power output and efficiency of turbine. The present paper deals with the study of effect of ambient air temperature on various parameters of 415 MW combined cycle power plant. The results of analysis reveals that for an increase in ambient air temperature by 40⁰C, the net power output of the gas turbine and combined cycle is found to decrease by 29.16% and 23.68% with 12.79% decrease in mass flow rate of inlet air. The efficiency of the gas turbine and combined cycle is found to decrease by 8.5% and 1.5% for the same increase in ambient air temperature.