Laser welding of the new nickel-based alloy K439B applied for engine turbine was carried out. The effects of post-welding heat treatment (PWHT) on the microstructure and mechanical properties of the joints were investigated. Semi-quantitative statistical results show that after PWHT, γ'''' phases in the whole joint grow up, and the radii of the primary and secondary γ'''' phases are about 25 and 150 nm, respectively. The results of mechanical properties show that PWHT improves the microhardness in the weld by about HV 100, and the critical resolved shear stress provided by γ'''' phases increases from 4.8 to 92 MPa. The tensile strength and yield strength of the heat-treated joint are comparable to those of the base metal. The presence of a large amount of carbides is the reason for the preferential fracture of the base metal during the tensile process.