Abstract: The 45B steel pin shaft used for chain tracks fractured early during use. The fracture reasons of the pin shaft were analyzed by means of chemical composition analysis, hardness testing, depth measurement of effective hardened layer, metallographic examination and fracture analysis. The results show that:the fracture type of the pin shaft was mixed of quasi-cleavage fracture and ductile fracture; the micro pores caused by peeling off of the spheroidal oxides formed the crack source during use; under the action of bending stress, the crack source expanded rapidly and resulted in the early fracture of the pin shaft.