Abstract: Quenching and tempering heat treatment of 4Cr5Mo2V steel with different carbon contents were carried out, and the effect of carbon content (0.35%, 0.47%, 0.71%, mass fraction) on its microstructure, carbide evolution and thermal stability was studied. The results show that There was no carbide in test steel with carbon mass fraction of 0.35% after quenching at 1 030 ℃, while granular vanadium-rich MC carbides existed in test steels with carbide mass fraction of 0.47% and 0.71%. M₂₃C₆ carbides were precipitated in test steels after quenching and tempering treatment, and the M₂₃C₆ carbides increased with the carbon content. After thermal stability test, a large amount of carbides were precipitated and coarsened, and with the thermal stability holding time decreasing and the carbon content increasing, the hardness and thermal stability of test steels increased. The increasing carbon content could improve the tempering transition temperature of test steels, the solid solution strengthening effect of carbon element and the precipitation strengthening effect of MC carbide were the main reasons for the enhancement of high temperature thermal stability of the steel.