Abstract: An early fracture occurred in the differential planetary gear of a vehicle during operation. The causes of planetary gear fracture were analyzed by means of macroscopic observation, chemical composition analysis, hardness test, metallographic examination and fracture analysis. The results show that the brass bushing in the gear had friction with the inner wall of gear, resulting in wear and abrasion of the inner wall of gear, and the fatigue fracture of gear was caused by stress concentration in scratch area. It is suggested that the abnormal friction of the gear should be avoided by checking the lubrication condition of the gear, the installation process, and whether there was foreign matter involved between the gear and the brass bushing.