A numerical method for analyzing and quantifying the thermal behavior of wind turbine nacelle of 2.5 MW operating in the extreme summer conditions have been presented for six cases. The effect of the external environment on the electrical equipment and mechanical components, the air flow and temperature fields within and around the nacelle were computed for the extreme temperatures of +45°C at the design wind speed of 12 m/s. For closed nacelle application without air-conditioning system the temperature within the nacelle varies from 340 K (67 °C) to 400 K (127 °C). For open system application without air-conditioning, temperature varies from 320K (47 °C) to 380 K (107°C) within the nacelle. The maximum temperature of 380 K (=123 °C) is obtained on the surface of the generator. Also, the temperature of 83 °C is obtained on the surface of the electrical box. The air conditioning system, 5 kg/s at 25 °C air supply, all cases except the DB with air conditioning, the surface temperatures of the components are approximately above the operation temperature limits. The simulation results have conformed that, to maintain an acceptable temperature levels inside the nacelle and on the component for typical summer conditions, the air conditioning system with some arrangements such as blades to increase the air circulation within the nacelle are needed. The air conditioning system of the nacelle has to be optimized and adjusted properly as functions of wind turbines rated power, external wind velocity and temperatures.