Abstract: The IN939 nickel-base superalloy is hot isostatic pressing (HIP) treated at varying temperatures, and the microstructure and phase are targeted of the alloy before and after the treatment. The evolution rules of morphologies, sizes and distributions of M₆C and γ' phases of the alloys treated at different temperatures are discussed, and the mechanical properties of the alloy are tested. The results show that after HIP treated at 1 220 ℃, the average length of the M₆C phase in the alloy is 3.71 μm, the area fraction is 1.22%, the average size of the γ' phase is 116 nm, with spherical and square morphologies. Under this condition, the yield strength of the alloy is maximum, and the standard deviations of tensile strength, yield strength and elongation after fracture are less than those of the samples treated at 1 188 ℃ and 1 250 ℃ respectively, show that the alloy has excellent and stable tensile property. Combining the microstructure analysis and mechanical property results, 1 220 ℃ turns out to be the most promising HIP temperature for IN939 alloys.