Abstract: The spline shaft of a starter generator fractured during service. In order to find out the failure mode and cause, macro and micro observation, metallurgical structure analysis, hardness testing, comparative analysis with intact serviced spline shaft and approving torsion test were carried out. The comprehensive analysis results show that the fracture mode of the spline shaft is overload under the combined tensile and torsional stress. The main fracture causing of the spline shaft is the higher hardness, lower torsional strength and plasticity. In view of the high hardness and poor impact resistance of the spline shaft, it is suggested to accuratelytailor the tempering temperature to control the hardness of the spline shaft at the lower limit of the current design requirements, or local ly temper the thread to improve the torsional strength and plasticity of the thread position, which could prevent such failures occur.