Abstract: The screw mandrel of large scale extrusion granulator fractured during operation. The causes of fracture were analyzed by means of fracture analysis, metallographic examination, chemical composition analysis and mechanical property test. The results show that the fatigue fracture occurred on screw mandrel with crack originating from the top of teeth. The main causes of fatigue crack were the surface indentation defects caused by the extrusion between the teeth top of screw mandrel and the screw element, and stress concentration at the corner of inner teeth of mandrel.