Abstract: In recent years, failure accidents occurred to plug valves used in drill pipes of oil wells frequently, such as seal failure, rotation failure, body fracture and so on. Aiming at a transversal fracture incident of one plug of the drill pipe in one well, the fracture causes of the plug valve were analyzed from the aspects of macroscopic analysis, chemical composition analysis, mechanical property testing, metallographic analysis and micro morphology analysis. The results show that: the fracture mechanism of the plug valve was fatigue fracture. The main reason of the fracture was that the impact toughness of the material was too low, resulting in the generation and extension of early cracks. In the end the corresponding measurements were proposed according to the failure reason.