Abstract: During the test of an engine accessory transfer gear box (ATGB), worn steps were observed on the tooth surface of the center-gear. In the present work, the causes and mechanisms of tooth surface wear were analyzed through macroscopic and microscopic observations, metallographic analysis, and hardness testing. The results show that the main cause of the rapid wear of center-gear tooth surface is the unqualified tooth surface roughness of the accessory gear, which resulted from coarse phosphide crystallization. The larger tooth surface roughness reduces the effective contact area of the mating tooth surfaces, increasing stress and thus leading to wear of the center-gear tooth surface with the relatively lower hardness. In addition, the unqualified fillet radius of the tooth top and tooth end of the accessory gear further accelerated the wear process. By shortening the phosphating time of the accessory gear, optimizing the nitriding process, adding surface finishing after nitriding, and improving the fillet radius at the tooth top and tooth end, the ATGB successfully passed the test.