Abstract: During grinding the cracks were created on the surface of carburized and quenched six square shaft manufactured with 20CrMnTi steel. The reasons were analyzed by means of macroscopic examination, metallographic examination and hardness test. The results showed that severely grinding burnt on the surface was originated from the repaired unsmoothly by abrasion wheel and inappropriate grinding parameters. When the tensile strength was less than the integrated stress during grinding, heating and cooling and phases transformation, the cracks emerging. Moreover, improvement measures were proposed.