Abstract: Aiming at the fracture failure of an automobile gearbox output shaft, the failure analysis was done to the output shaft through macroscopic trace analysis, fracture analysis, chemical composition analysis, metallographic examination, hardness test and hydrogen content test. The results show that the fracture of the gearbox output shaft was a kind of delayed fracture. The tempering martensite structure with high hardness of the carburized layer in the threaded end of the output shaft provideds ‘necessary condition’ for the delayed fracture, which was caused by the inappropriate local processing of the screw thread. And a certain foreign static tensile stress, its own residual stress and hydrogen content provided ‘sufficient condition’ for the delayed fracture. Finally, some improving and preventive measures were put forward.