Abstract: The transmission shaft of a truck fractured at the welding joint of transmission shaft tube and universal joint fork. To find out the fracture mode and causes, the chemical compositions of the transmission shaft tube and the universal joint fork were tested, the fracture surface and microstructure of the welding joint were observed, and the hardness of the welding joint was also tested. The results indicate that the fracture mode of the welding joint of transmission shaft tube and universal joint fork is fatigue fracture. The fatigue crack initiated from the stress concentration zone near the welding fusion line of universal joint fork which is located at the bottom of the V-type weld. The martensite existing at the heat affected zone of universal joint fork accelerated the initiation and propagation of the crack. Based on the fracture cause, the welding technology has been improved and the chemical composition of the transmission shaft tubes has been strictly controls. As a result, the type of failure has been effectively prevented.