Abstract: The cracking reason for the principal axis of an elevator was fatigue fracture according to the failure analysis results.Upper bainite in microstructure and lower hardness decrease the fatigue intensity.The elevator works with high frequency during the process of “start up” and “stop”.So the “R” transition of keyway is the most stress concentration position and the minimal loaded area and thus become the cracking source,which led to the final fatigue fracture and failure of principal axis of the elevator.