Abstract: The SCM435 steel rocker bolt of an automobile engine fractured during the test. The causes of bolt fracture were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, hardness test, scanning electron microscopy analysis. The results show that the chemical composition and hardness of the rocker bolt met the product requirements, and the microstructure was normal. The rocker bolt was eccentrically loaded during use, and it bore abnormal bending stress exceeding its fatigue strength, which eventually led to fatigue fracture of the material.