This article took K439B alloy with a temperature bearing capacity of 800 ℃ as the research object to study the γ′ phase morphology in the standard heat-treated condition, and the γ′ phase evolution and coarsening behavior during aging at 800 ℃ for 6000, 7000, 8000, and 10 000 h. The results indicate that the γ′ phase at the dendrites in the alloy is fine and uniform, while the γ′ phase between the dendrites presents two types. After long-term aging, the bimodal γ′ phase is still retained in the interdendritic region, and the γ′ phase transforms from spherical to cubic shape without any rafting phenomenon. During the long-term aging process, the γ′ phase size of the K439B increases, but the change in volume fraction is not significant. The variation of γ′ phase size with aging time conforms to both Lifshitz-Slyozov-Wagner (LSW) and theory of interface diffusion control (TIDC) models. The internal reason is believed to be the equivalent diffusion rate of elements between the matrix and interface.