Abstract: The 20MnCr5 steel gear shafts was treated with carburizing and quenching. After fine machining, obvious spotted scars were found on the surface of the gear shafts. The causes of surface defects of the gear shafts were analyzed by means of macro observation, chemical composition analysis, scanning electron microscopy analysis, metallographic examination and so on. The results show that the copper in the anti carburizing coating used before carburizing heat treatment penetrated into the grain boundaries of steel and caused embrittlement cracking of steel material. Then other low melting point substances in the anti carburizing coating penetrated into the grain boundaries of steel and destroyed the continuity of the steel matrix. Therefore, the surface of gear shaft appeared spotted scar defects after fine machining.