Abstract: T91 steel was welded by simulated three-temperature process of transient liquid phase diffusion welding, and then tempered at 780℃ for 2 h. The microstructure and mechanical properties of the heat-affected zone and the effect of tempering treatment on the mechanical properties of the heat-affected zone were studied. The results show that the heat-affected zone was divided into four zones:overheated coarse lath martensite zone at 0-7.5 mm distance from the weld, fine lath martensite zone at 7.5-22.5 mm distance from the weld, ferrite+martensite zone at 22.5-37.5 mm distance from the weld and tempered martensite zone at 37.5-42.5 mm distance from the weld. With increasing distance from the weld, the hard and brittle tendency of the heat-affected zone decreased and the fracture mechanism changed from brittle fracture to ductile fracture. Compared with those before tempering, the tensile strength and hardness of the heat-affected zone after tempering decreased but the toughness increased.