Abstract: Failure analysis on a connecting bolt of electric locomotive was carried out by macro and micro observation, chemical composition analysis, properties testing and microstructure examination. The results show that the bolt presented the feature of brittle fracture. The crack source was located at the transitional region between screw and hexagon head. Intergranular and quasi-cleavage fracture feature was found at the source region of the fracture surface, and there were some holes and tearing ridges at the grain boundary surface. These characteristics are the same as those of hydrogen brittleness fracture. In addition, there existed excessive corrosion at the transitional region between screw and hexagon head. Excessive acid pickling caused hydrogen to permeate into the matrix of the bolt. As a result, hydrogen induced delay rupture occurred to the bolt under the coeffect of torsion stress and tensile stress.