Abstract: Multiple fractures in short time happened to a type of diesel engine push rods in the process of test, and means such as macroscopic analysis, chemical compositions analysis, metallographic examination and fracture analysis were used to analyze the fracture reasons of the push rods. The results show that the main cause for the fracture was that the undercut of the push rod was designed for inner R form, at the same time the transition arc radius was rather smaller and the machining quality of the R position was poor. When working, the R position would form serious stress concentration and produce fatigue cracks which continuously expanded until the push rod fractured.