Abstract: A certain coal mining machine's planetary gear developed cracks after carburizing and quenching, resulting in product scrap due to the deep location and depth of the cracks. To analyze the failure cause of the planetary gear, we conducted an analysis and discussion on the raw materials, heat treatment process, microstructure, grain size, as well as the macro and micro features of the cracks. The results show that the chemical composition of the material used for the planetary gear meets the standard of 18Cr2Ni4WA steel. The cracks are typical quenching cracks along grain boundaries, and their main cause is related to quenching stress. Through improvement of the heat treatment process and control of the heat treatment process, no tooth surface cracks have been found subsequently.