Abstract: The effects of different deep cryogenic holding time under liquid nitrogen temperature（-196 ℃） on the microstructure and hardness of W6Mo5Cr4V2 high speed steel were investigated with metallurgical microscope, scanning electron microscope（SEM） and hardness tester. The tempering treatment at different temperatures on the samples after 2 h deep cryogenic treatment was conducted. To compare with the traditional treatment sample, the effects of different tempering temperatures on the microstructure, hardness and red hardness were investigated. The results show that, extending the deep cryogenic treatment time is conducive to improve the microstructure of high speed steel and the hardness increases. The amount of secondary carbides precipitation is the most and uniform distribution in the sample after 20 h deep cryogenic treatment, and the hardness increases significantly. To a certain extent, the hardness of deep cryogenic treatment samples increase with the increase of tempering temperature, and the carbides distribute uniformly and microstructure refine. After the holding treatment at 600 ℃and 625 ℃, the red hardness of the samples after 2 h deep cryogenic treatment with 100 ℃ tempering treatment for 1 h are better than that of traditional process treatment sample.