A viscosity index improver was produced by using the J0010 ethylenepropylene copolymer (OCP).Through high temperature test,crankcase simulation test and road test,the influence of high temperature degradation on the viscosity maintenance property of the viscosity index improver of OCP was studied.The results show that the main reason of viscosity drop of J0010 under high temperature is due to the thermal degradation of J0010 under high temperature,and the result of a long time high temperature test by crankcase shows less relationship with the road test.Both the low temperature property and high temperature viscosity maintenance property of the viscosity index improver are improved after high temperature degradation.The high temperature degradation speed of viscosity index improver is faster as contrasted with road test.