Abstract: Grinding hardening test of 42CrMo steel in quenched and tempered state was carried out by taking liquid nitrogen as cooling fluid, and then, the tested steel was treated respectively by air cooling and deep cooling.The microstructure, hardness and surface roughness of strengthened layer were studied, and the microstructure of strengthened layer was compared with that of the strengthened layer with emulsion cooling grind-hardening. The results show that, after liquid nitrogen cooling grind-hardening, martensite in strengthened layer was more tiny comparing with that in strengthened layer with emulsion cooling grind-hardening; After deep cooling treatment,the numbers of martensite and carbide in hardening layer increased; After liquid nitrogen cooling grind-hardening, with the increase of grinding depth, the thickness, highest microhardness and surface roughness of the strengthened layer were all increased.