Abstract: A fracture accident of a screw shaft of a polypropylene extruder occurred during operation. In order to find the fracture reasons, the failure analysis was carried out by means of macroscopic fracture analysis, scanning electron microscope observation, chemical composition analysis, microstructure observation and mechanical property testing. The results show that torsional fatigue was the cause of the fracture, and the cracks originated from the spline root. Low fatigue strength, stress concentration at spline root, micropore aggregation and uneven microstructure were the reasons together caused the screw shaft fatigue fracture in the early service life. Some measures, including improving heat treatment process and installation precision and examining surface defects, were suggested.