The hardness, tensile strength and impact toughness of one quenched and tempered steel with nominal composition of Fe0.25C-3.0Cr-3.0Mo-0.6Ni-0.1Nb (mass fraction) both at room temperature and at elevated temperatures were investigated in order to develop high-strength steel for long-life gun barrel use. It is found that the steel has lower decrease rate of tensile strength at elevated temperature in comparison with the commonly used G4335V high-strength gun steel, which contains higher Ni and lower Cr and Mo contents. The high elevated-temperature strength of the steel is attributed to the strong secondary hardening effect and high tempering softening resistance caused by the tempering precipitation of fine Mo-rich M2C carbides in the α-Fe matrix. The experimental steel is not susceptible to secondary hardening embrittlement, meanwhile, its room-temperature impact energy is much higher than the normal requirement of impact toughness for high strength gun steels. Therefore, the steel is suitable for production of long-life high-strength gun barrels with the combination of superior elevated-temperature strength and good impact toughness.