Abstract: The bolt of front brake hose of an automobile engine was broken. The causes of bolt fracture were analyzed by means of macroscopic observation, chemical composition analysis, scanning electron microscope analysis, metallographic examination and hardness test. The results show that the bolt was not heat treated, resulting in cold hardening was not eliminated. In the pickling and electroplating process, hydrogen ions were immersed in the root of the bolt, causing hydrogen induced delayed fracture at the root of the bolt.