Three kinds of γ′ phases with different morphologies and sizes were obtained by changing the primary aging temperature (1150, 1180 and 1200 °C) in the heat treatment process. Then, the samples with different γ′ morphologies were tested under condition of intermediate temperature and high stress (760 °C/800 MPa). The results show that the size of γ′ phase plays an important role in the stress rupture property. Increasing the size of γ′ phase appropriately can promote the uniform deformation of the alloy, thus improving the stress rupture life of the alloy.