Abstract: In order to achieve good toughness and strength in welding 10Ni5CrMoV steel, heat treatment is usually carried out to improve the mechanical properties of the weld metal. 10Ni5CrMoV steel was welded with rich Ar shielding gas, and the microstructure and mechanical properties of the weld metal were investigated after different heat treatment. The results indicate that the microstructures of the weld metal are bainite, martensite and a small amount of retained austenite. After quenching and tempering, the microstructure of the weld metal is tempered martensite. The amount of retained austenite in the weld metal decreases and the size of carbide in the martensite increases with tempered temperature increasing. The strength of the weld metal decreases as tempered temperature increases, and the toughness is improved when tempered temperature is increased.