Abstract: The 50Cr steel bar after quenching and tempering heat treatment fractured when turning into steel shaft. The fracture causes of steel shaft were analyzed by means of chemical composition analysis, metallographic examination and hardness test, combined with processing and heat treatment process. The results show that brittle fracture occurred in the 50Cr steel shaft. The main reason was the formation of upper bainite during quenching and tempering heat treatment, which reduced the mechanical properties of material. The processing trace on the surface of steel shaft and the stress concentration at the root of step were the inducing factors for brittle fracture.