Abstract: The 300M steel underwent cryogenic treatment with different cryogenic temperature preceded by quenching and followed by tempering. Afterwards, its hardness and wear resistance were tested, the microstructure was observed using scanning electronic microscope, chemical composition in microzone was examined by energy dispersive spectrometer, and the phase composition was analyzed with X-ray diffractometer, so as to investigate the effect of cryogenic treatment on the hardness and wear resistance of 300M steel, as well as its microscopic mechanism. The results show that the hardness of 300M steel with cryogenic treatment insignificantly changed, its wear resistance improved and the friction coefficient decreased. Furthermore, both the wear resistance and friction coefficient of 300M steel with cryogenic treatment at −160℃ improved more than those with cryogenic treatment at −80℃. It is mainly owing to the shrinkage of martensite cryogenic treatment, the more saturation and segregation of carbon atoms at dislocations in martensite, and then the more dispersed precipitation of fine carbides during tempering, thereby enhancing both wear resistance.