Abstract: The fastening stud of diesel engine in a nuclear power unit broke during reassembly. The causes of stud fracture were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, hardness test, scanning electron microscope analysis and energy disperse spectroscopy analysis. The results show that the quenching crack was formed in stud due to the improper quenching temperature control and the improper quenching speed. Under the action of the installation torque in the process of reassembly, one-time overload fracture occured at the quenching crack of stud.