Abstract: A fracture accident of screw shaft of a twin-screw extruder occurred during operation. In order to find the fracture reasons, the failure analysis was carried out by means of macroscopic fracture observation, scanning electron microscope observation, chemical composition analysis,microstructure observation,mechanical property testing and finite element simulation. The results show that torsional fatigue was the cause of the fracture, and the cracks originated from the tooth root, with several fatigue crack sources. Stress concentration at root of the tooth; poor thermal treatment process leads to microporous aggregation and uneven microstructure, these factors were the reasons together caused the screw shaft fracture in the early service life.