Abstract: In a factory, the 40Cr steel bolt for tension wheel of vehicle fractured during trial installation. In order to confirm the fracture reason, the failure analysis was conducted on the bolt. The results show that the fracture mode of the bolt was hydrogen induced delayed fracture. The fracture reasons were that the hardness of the bolt was relatively high due to too many tempered martensite in the microstructure and that the micro-cracks existed in the bolt; The hydrogen introduced during the pickling and galvanizing process diffused into the cracks, resulting in the fracture. To control the bolt hardness in 32-34 HRC during machining process and eliminate the hydrogen by baking at an appropriate temperature for a sufficiently long time will be in favor of preventing the hydrogen induced delayed fracture of bolt.