The performance of a radiator is generally determined using a wind tunnel, in which the air velocity is uniform. However, when it is installed in a car, the distribution of the air velocity becomes nonuniform due to front-end openings, cross members, and horns etc., resulting in lower performance. In this study, several underhood flow simulations have been first performed to get flow rates and velocity distributions over the radiator. Secondly heat release rates are calculated by both a performance curve and a radiator model. Finally, using an engine cooling system simulator, radiator-top-tank temperature is predicted and the variations of heat release rate and radiator-top-tank temperature with nonuniformity of air velocity distributions are analyzed. The results show that the current engine cooling model successfully accounts for the nonuniformity effects that should be considered for higher accuracy in predicting engine cooling performance.