The effects of V content (0.1wt%, 0.2wt%) on the carbide evolution and mechanical properties of ultra-clean 30Cr2Ni4MoV rotor steel under different heat-treatment states (as tempered and as step cooled) were investigated by SEM, EBSD, XRD, TEM, and APT. The results show that both tempered steels show lath martensite microstructure. The increase in V content has no obvious effect on the carbide type (M₂₃C₆, M₂C and MC) and size, but promotes the precipitation of more and finer V-riched carbides MC, which refines the prior austenite grain size of the 0.2V steel. The refinement of grain size and precipitation of finer MC carbides increase the yield strength of the 0.2V steel by 147 MPa through grain refining strengthening and precipitation strengthening. After step cooling heat-treatment, the microstructures and the type of carbides in both steels remain stable and the size of carbide grows slightly. Meanwhile, the yield strength of them shows a slight decrease due to the carbide coarsening. As for 0.2V steel, the mobility of dislocations decreases due to precipitation of more MC carbides, which induces the decrease in critical stress σ_f of crack propagation and promotes the tendency of crack initiation and propagation. Therefore, compared with that of 0.1V steel, the fracture appearance transition temperature of 0.2V steel increases by 21 ℃.