Abstract: The as-cast microstructure and properties of K465 nickel-base cast superalloy are investigated, as well as they are during heat-treatment. The results showed that the alloy as-cast is composed of γ phase, γ′ phase, (γ+γ′) eutectic and MC carbide, and its average tensile strength and percentage elongation at ambient temperature are 960 MPa and 6.0%, respectively. The creep rupture life under the condition of 975°C/230 MPa is 28.1 h. After the 1210°C/4 h solution treatment followed by air-cooling, the crystal boundary MC carbide partly becomes M6C one with γ′ phase particle size reduced to 0.1-0.2 μm and the tensile strength and elongation at ambient temperature up to 1055 MPa and 4.0%, respectively. The creep rupture life is 50.3 h under the same condition. With 0.02 wt% Mg added in, the grains of MC carbide in the alloy is spheroidized, and the average tensile strength and elongation of the alloy at ambient temperature are 990?MPa and 5.0%, respectively, with a creep rupture life up to 56.3 h under the same conditon.