Abstract: The GCr15 transmission shaft with a product specification of φ50 is broken during straightening after medium frequency induction hardening. The fracture, microstructure, microstructure, hardness and chemical composition of the broken transmission shaft were analyzed. The nature and reason of transmission shaft fracture are determined. The results show that the fracture crack source of the transmission shaft has the characteristics of intergranular dimple fracture. The fracture failure property is brittle fracture; the raw material banded and reticulated carbides of GCr15 transmission shaft exceed the standard; the medium frequency induction quenching temperature is too high. The surface and core hardness of the transmission shaft are high, the surface overheating leads to coarse martensite structure, which increases the brittleness of the transmission shaft. Under the superposition of quenching stress and straightening stress, the residual stress in the transmission shaft increases greatly and causes cracks, which is the main reason of straightening fracture. According to the cause of the fracture, some suggestions are given, such as optimizing medium frequency induction hardening process, strengthening raw material purchase and re-inspection management, improving straightening process, etc.