Inlet air cooling of gas turbine is one of the most probable improvements in power station especially in Egypt. For example in north Cairo power station which has four gas turbines the rating output of one of them is 250 MW but at 45ºc atmospheric temperature the output is 230 MW and at 16ºc atmospheric temperature is 260 MW this different is enough for region as Aswan. The main effect of increasing atmospheric temperature is the decrease of inlet mass flow rate, increasing compressor work and decreeing turbine work .All of these decrease output power. For diesel engine the effect of inlet air temperature is exist but less than gas turbine although the same process occurs in both but in diesel engine the effect of compression is more less than gas turbine and the other process are sophisticated . So we change inlet temperature from 25ºc to 50ºc and we measure the performance of diesel engine at these temperatures. In diesel engine the most benefits is the decreasing of fuel consumption at constant load at decreasing inlet air temperature especially at full load.