Abstract: Cracks were found on the tooth surface of a worm after grinding.Optical emission spectrometer,micro-hardness instrument,optical microscopy and scanning electronic microscopy were employed to analyze the cracks reason.The results show that the microstructure and microhardness of the carburized layer on the worm tooth surface cracked after grinding were changed obviously.The cracks on the tooth surface of worm were radial and went along rain boundaties.The direction of cracks was perpendicular to grinding trace approximately,the cracks depth was less than 0.42 mm.The cracks on the worm tooth surface were caused by improper operation of grinding.Some grinding process was improved crack no longer appeared.