The effects of deep cryogenic treatment in different holding times combined with the traditional HSS heat treatment processes on the microstructure and hardness of M42 HSS were investigated with metallurgical microscope, SEM and Hervey hardness tester. The process included: quenching+deep cryogenic treatment, quenching+deep cryogenic treatment+tempering and quenching+tempering+deep cryogenic treatment. The results have shown that deep cyogenic teatment in 24 h after quenching can obviously refine grain size, improve the hardness of M42 HSS,and significantly promote the transformation of retained austenite to martensite and the precipitation of dispersion distribution carbide, and meanwhile result in the change of the shape of martensite which is different from the conventional heat treatment.Deep cryogenic treatment before tempering can reduce the temperature of secondary hardening, where the peak temperature declined to 450 ℃ compared with 525 ℃ of the process without the deep cryogenic treatment. The hardness of secondary hardening peak is 998.2HV,which is 5% higher than that of the process without the deep cryogenic treatment. The effect of deep cryogenic treatment after tempering on the microstructure and the hardness of M42 HSS is not obvious.