Abstract: The motor shaft of a finishing mill was broken when plunged into test-run for one month.The fracture causes were analyzed by means of metallographic examination,chemical composition analysis,mechanical property test and slow strain rate test,etc.The results show that the crack feature was cleavage fracture,and the existing residual thermal stress(residual tensile stress) in the motor shaft center resulted from large size,delayed heat treatment and abnormal heat treatment processing was the main cause inducing the motor shaft fracture.On the other hand,the poor toughness in serions composition segregation zone of the motor shaft forging center played a certain promotion role in crack initiation and propagation.