The effect of tempering temperature on strength and toughness of the novel carbide free bainite/martensite (CFB/M) duplex steel was investigated. The results showed that the CFB/M mixed microstructure has better tempering stability than single martensite microstructure. The steel has good combination of strength and toughness after tempering at 350 ℃, because the CFB in CFB/M mixed microstructure separates the original austenite grains and causes the refinement of lath martensite, and thermal and mechanical stable retained austenite in CFB is deformed absorbing energy and blocking crack propagation.