Abstract: The valve spring in an automobile engine fractured. The present work aims to identify the failure mode and cause by visual inspection, hardness measurement,metallurgical analysis, chemical composition analysis, fracture appearance observation, and EDS analysis. The results show that the fracture mode of the valve spring is fatigue fracture. The direct cause of fracture is that there are pits at the surface of the valve spring. The pits had existed when the spring was in the form of steel wire. Based on the results, the valve spring supplier has conducted improvement measures, including adding an automatic recognition system of pits before spring coiling machines and increasing the measurement frequency. After that, the problem of fracture of valve spring has been effectively solved.