Abstract: 20Cr axes of unilateralism organ had broken during short time test, the cracking reason was analyzed by means of chemical composition analysis, macro and micro examination.The rerults showed that, carburized layer＇s depth was too much, the degree of reticulation carbide and martensite were exceed standard seriously. The overhigh carbon potential and overheating of heat treatment led to quenching cracking, wihch was the central reason for the cracking and failure of the unilateralism organ.