Abstract: Black hexagon bolts used in engine base were ruptured after installation for 24 h. In order to find the failure reasons, the optical microscope, scanning electron microscope, direct reading spectrum, and hardness tester were used to analyse microstructure, hardness and chemical composition. The results show that surface hardness and core hardness of the failure bolts were 518, 435 HV0.3, respectively. Microstructure of the bolts were tempering sorbite, and the macrograph of the bolts existed light area and micro-morphology existed chicken-feet-shape. The intergranular crack in the root of thread was not bifurcate, and which showed that the bolts was typical hydrogen brittleness fracture. In order to avoid such case rehappening, strictly control the pickling and heat treatment process were needed, and dehydrogenation was necessary for the bolts.