Abstract: The high temperature bolt of a steam turbine fractured during the operation of the unit. The fracture causes of the high temperature bolt were analyzed by means of macro observation, chemical composition analysis, mechanical property test, macro structure inspection, metallographic examination and fracture analysis. The results show that the grain size of the high temperature bolt of the steam turbine was coarse, and it had been in service under high temperature and high pressure for a long time, resulted in the gradual precipitation of carbide and the decrease of impact toughness of the material. Under the impact stress caused by unit start-up, shut-down and unit load fluctuation, the high temperature bolt finally had brittle fracture.