Abstract: This paper establishes a new method of obtaining the performance of an engine oil cooling system.The method involves the equation of heat gain of the oil passing through an engine and performance calculations of oil heat exchangers.When the heat exchange between oil and ambient in the oil heat exchanger cannot be ignored,the complex heat transfer processes could be treated as two independent approaches,heat transfer from oil to cooling fluid as well as from coolant,to ambient.Each separate approach is a simple heat transfer problem for which an analytic solution can be acquired easily,and then the real outlet temperature value of oil is obtained by a linear combination correlation of these two solutions.Based on the theoretical analysis,a more effective NTU correlation of the heat exchanger is deduced under the operating condition.This method has been well verified by the experimental data of an engine and can be used to calculate the performance and estimate the heat-sinking capability of the oil cooling system of engine in severe condition.