Abstract: The abnormal fracture happened to the transition section of gripping head during the tensile test of 300M steel specimens. The specimens were investigated by means of chemical composition analysis, macrographic examination, metallographic examination, hardness test and SEM analysis, in order to find the causes of the abnormal fracture. The results show that: the abnormal fracture of the specimens was related to the generation of carburization phenomenon caused by the heat treatment, and the unreasonable design of the transition section of the gripping head.