Abstract: The U-shaped bolt for fixing the balance suspension and axle of a newly developed vehicle broke after a period of road test. Based on the method of physical and chemical inspections, the analyses were performed on the macroscopic morphologies, microscopic characteristics, chemical compositions, mechanical properties, hardness, metallographic structures, and elements on the fractured bolts surface to determine the type and mechanism of bolt fracture, and then explore the cause of fracture. The results indicate that the fracture type of the U-shaped bolt is high cycle fatigue fracture. The interference between the R angle of the U-shaped bolt and the backing plate led to the surface stress concentration at the R angle. The decarburization on the surface of the U-shaped bolt caused the initiation of micro crack defects at the R angle. Under the action of alternating stress, the cracks gradually expanded, and ultimately led to the fracture and failure of the U-shaped bolt. Hence, the formation of cracks could be avoided by repairing the mold, optimizing the R-angle size of the U-bolt, and strictly controlling and improving the heat treatment process to prevent decarburization.