Abstract: Fracture reasons of the fastening screws used for high-pressure part in a microwave oven were analyzed by means of macro analysis, metallographic examination and fracture analysis. The results show that center carbon content of the screw increased during carburizing process, so tempered low-carbon martensite structure formed after thermal quenching and tempering treatment, which caused high hardness and brittleness of the screw center. At the same time, the effect of surface dehydrogenation was bad. So hydrogen embrittlement delay fracture happened to partial screws under action of the service stress.