Abstract: A 45 steel reducer shaft fractured during use. The fracture properties and causes were analyzed by means of macro analysis, chemical composition analysis, mechanical property test, metallographic examination and fracture micro analysis. The results show that the fracture property of the reducer shaft was multi-source fatigue fracture, and the fracture were mainly caused by the lower mechanical properties because of improper heat treatment (no quenching and tempering treatment), the machining defects on the shaft surface and the serious stress concentration at the shaft diameter changing position.