A new casting Ni-based superalloy is used in industrial and aircraft turbine because of its high strength and excellent hot corrosion resistance at high temperatures (about 900 ℃). The effect of heat treatment process on its microstructure and mechanical properties of the experimental casting Ni--based superalloy was studied in the present investigation in order to improve its application level. The results showed that the stress--rupture lives were changed with the increase of heat treatment temperatures. After solution treated for 2 h, then cooling by air cooling, the stress-rupture life under the conditions of 760 ℃ and 660 MPa was the highest in the case of heat treatment temperature at 1220 ℃, while under the testing conditions of 980 ℃ and 180 MPa the stress--rupture life was the highest in the case of heat treatment temperature at 1180 ℃. The stress--rupture life decreased when being heat-treated at temperatures above 1220 or 1180 ℃. The tensile strength of the alloy was improved with the increase of heat treatment temperatures from 1120 to 1220 ℃, and was decreased in the case of heat treatment temperature at 1240 ℃. The experimental results also showed that with the increment of holding time from 2 to 8 h at 1120 ℃ the stress--rupture life increased under the conditions of 760 ℃ and 660 MPa, while it was decreased under the conditions of 980 ℃ and 180 MPa. It was found that the tensile strength was higher for the alloy being held for 2 and 4 h than that for the alloy being held for 6 and 8 h, and the stress--rupture lives changed with the different cooling ways. The mechanical properties of the alloy may be determined by the size, shape, distribution and volume fractions of γ' phase and γ/γ' eutectic.