Abstract: The physical and chemical test and failure analysis were done to the BM4 output shaft which fractured during service. The results show that the fracture of the output shaft was mainly caused by the serious insufficient hardenability during the carburizing and quenching process, which resulted in the existence of a lot of granular bainite and blocky ferrite in the matrix microstructure and a considerable reduction of strength of the material. At the same time, the presence of a large number of non-metallic inclusions in the raw material also increased the brittleness of the material. The serious banded structure further reduced the transversal tensile strength of the material. The combined action of the above mentioned factors ultimately led to the early fracture failure of the output shaft in bearing.