Abstract: The fracture reason of electric motor shaft of 45 steel was analyzed by means of chemical composition analysis, mechanical property testing, metallographic examination, fracture analysis and so on. The results indicated that there was stress concentration at the root of knuckle diameter of the shaft, where had small curvature radius and large machining roughness; the intensity of the shaft was not enough with the annealing treatment and the high content of MnS in the steel led to the decrease of the mechanical properties, which resulted in the fatigue fracture of the shaft at alternate loading.