Abstract: In this paper, a failure analysis of the plate gear is made by means of macroscopocal fracture observing, hardness testing, chemical composition analysis, microstructure examining and SEM. The result shows the things as follows-Firstly, generally there is approximate 25μm black structure and more residual austenite in the carburized layer. Secondly, the obvious cuter marks exists in the surface and trough of tooth; And last, a more obvious undercut occurs in the root of tooth. All these defects of above notably decrease the fatigue strength of the gear and are internal cause of the early stage fatigue failure of the plate-gear. Among others, the cuter marks and the black structure in the cuter marks is mainly cause. Because all defects of above has existed, when it serves at suffering a greater outside force, the early stage fatigue failure of plate-gear occurred.