The microstructural evolution of a nickel-based heat resistant alloy during long term aging (10000 h) at 700 °C was systematically investigated. Experimental results showed that the main precipitates after pre-aging treatments were MC, M23C6 and homogeneous γ’. The weight fraction of γ′ increased continuously with the aging time up to 5000 h, while an obvious coarsening was observed after aging for 100 h. The grain sizes of γ′ precipitates from all aged samples exhibited unimodal distributions, and their coarsening behavior showed good agreement with the LSW model. Compared to the MC carbides, the weight fraction of the M23C6 carbides increased significantly with the increase of the aging time. This could be attributed to their precipitation from the supersaturated matrix. Due to the precipitation and coarsening of γ′, the hardness of the aged samples increased sharply during the first 100 h of aging and then decreased with the increase of the aging time (up to 10000 h). •All aged samples were quantitatively studied by the phase analysis.•The sizes of γ′ precipitates from all aged samples exhibited unimodal distributions.•The increase of M23C6 during the aging time could be attributed mainly to their precipitation from the supersaturated matrix.