Abstract: The intermediate shaft for certain car transmission has ruptured in durability test. The chemical composition analysis,metallography examination,scan electricity lens observation and hardness measurement for failed parts have been conducted.All above results show that decrease of carrying capacity for intermediate shaft gear resulting from not over depth of non-quenching part of tooth root surface and not good hardness distribution but low surface hardness,caused bending fatigue overload rupture in durability test. Some improvement comments were given out.