Abstract: Anhui Jianghuai Automobile Co.Ltd.Technical Center, Hefei 230601 ChinaA test approach of automobile rotating parts'effective mass is proposed to improve the test accuracy of drum dynamometer.Firstly, the drum simulation mass and the force loaded on the tire by the drum after loss compensation were determined by comparative tests on a drum dynamometer.Then the effective masses were tested for a truck and a passenger car.The results show that the accuracy of the heavy truck is up to 1% based on the obtained 7 groups coast down data, and the accuracy of car is up to 4% based on the obtained 12 groups coast down data.So we can make a conclusion that increasing the test times can effectively improve the statistical accuracy, and the larger the inertia of tires, the higher the relative accuracy of tests if the respective test times are the same.The determination and compensation of automobile rotating parts'effective mass could effectively improve the road simulating test accuracy of drum dynamometer.The study in the paper provides the theoretical and experimental basis for drum dynamometer test.