Cr8–type cold work die steels have been widely used in recent years. These steels have obvious secondary hardening effect due to high V content. Now it has been successfully used in high speed steel that Nb is substituted for V. However, it has been not widely applied in cold work die steel yet. In order to investigate the effect of different Nb–V contents on the microstructure and mechanical properties of this kind steel and the role of Nb in tool and die steels, in this paper the microstructure and morphology of three Cr8–type cold work die steels with different Nb–V contents were investigated by OM, SEM and EDS analysis, the mechanical properties including the hardness and impact toughness were measured at room temperature. Experimental results indicate that the precipitation temperature of MC–eutectic carbides increase with the increase of Nb content, and the improvement of their properties is closely related with Nb content. Moreover, the increase of Nb content causes the increase of metatectic and eutectic temperature and the disequilibrium of C distribution in primary γ phase. The granular carbides appear near the grain boundary of primary  phase when Nb content reaches to 1.32%; the variation in Nb/V ratio with the increase of Nb content will change the type of MC–eutectic carbides, the type of MC–eutectic carbides from mainly VC change to VC and a small amount of (Nb, V)C and then to mainly NbC and (Nb, V)C. So the shape of ledeburite is more straight. If Nb content reaches to 1.32%, the substitution of Nb for V increases quenching hardness, the peak of quenching hardness moves to low temperature region; the substitution of Nb for V is beneficial for secondary hardening in tested steels, and higher hardness and temper softening resistance can be obtained in them. In addition, their impact toughness decreases with the increase of Nb content.