Abstract: The fracture surface of a failed injection valve nut of diesel engine high pressure oil pump was investigated by visual and SEM.The microstructure of the material for the failed injection valve nut was observed.The results show that fatigue fracture had taken place on the injection valve nut.Absence of the transition fillet at the root of the cylinder interior of the failed injection nut should be mainly responsible for the failure of the injection valve nut and the rough machining in the root region was related to the fracture of the injection valve nut.