Abstract: The fracture reasons of the motor shaft of a polypropylene pellets homogenizing fan were analyzed from the aspects of fracture macro and micro characteristics, chemical compositions, mechanical properties, microstructure and so on. The results show that the fracture of the motor shaft belonged to brittle fatigue fracture. Defects such as loose pores and intergranular micro-cracks and no quenching and tempering treatment, made the mechanical properties of the motor shaft material insufficient, which was the internal factor for the fracture of the motor shaft. The overlap of machined tool marks and material stratification defects at the lower part of the motor shaft, led to stress concentration and multi-source fatigue, which was the external factor for the fracture of the motor shaft.