Abstract: Flexible shaft of the starter-generator for a plane fractured early after serving for 160 h. In order to find out the fracture cause, fracture observation, wear trace analysis, hardness test and microstructure analysis were carried out. The results show that the fracture mode of the flexible shaft was low cycle fatigue failure under high stress. The main reason for the flexible shaft fracture was that the abnormal bending stress was caused by the dislocation of the flexible shaft of engine and generator as link dislocation of end cap and ring flange. The stress superposition of the mormal distorting and abnormal bending stress which exceeded the fatigue strength of the shaft. The fracture of the flexible shaft could be effectively prevent by improving the height of the dowels.