Abstract: The double head stud of high pressure main steam valve of a thermal power unit fractured during maintenance. The fracture reasons of the stud were analyzed by means of macroscopic observation, chemical composition analysis, scanning electron microscope analysis, metallographic examination and mechanical property test. The results show that the grain of the stud was coarse, the impact toughness was low, the service time was long, and the abnormal structure of the black mesh austenite grain boundary was produced, which led to the embrittlement of the material, the notch sensitivity and the embrittlement tendency were increased. During the overhaul, the stud was brittle fracture due to the impact force of disassembly.