Abstract: Here were analyzed the macroscopic and microscopic fractures, metallographic structure, chemical components and hardness of the exhaust valve of diesel engine of a certain type. The results showed that the upside of the exhaust valve rod fractured first and the underside ensued. The fracture of exhaust valve was fatigue fracture, and the original fracture region was located in the inside of the friction welding position. The main reason for the fatigue fracture of the exhaust valve was the existence of carbon inclusion which led to the generation of fatigue source in the welding seam.