Abstract: The flexible 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, and hardness testing were carried out. It is found that the failure mode of the flexible shaft is torsion fatigue. The main fracture cause of the shaft is the mismatching between the flexible shaft and the rotor hollow transmission shaft. The flexible shaft transmitted the torque by means of conical surface joint. However, it is hard to reliably guarantee the conical surface joint proportion during the production and maintenance processes, leading to poor process controllability and low reliability of products, as well as the problem of unproper structure design. The suggestion that the flexible shaft should change the conical structure to spline structure was proposed to prevent such failures.