Abstract: Different heat treatment processes were performed to 25MnV steel used for petroleum casing pipe, and the effect of subcritical quenching process on microstructure and properties was studied. The results show that compared with the conventional heat treatment, the 25MnV steel after subcritical quenching at 800 ℃ and tempering could got a composite microstructure of ferrite strip distributing between lath martensites, and the impact toughness was significantly improved although the strength decreased a little.