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To clarify the initial microstructure dependent γ′-coarsening behavior, a corrosion-resistant Ni-based superalloy with the varied γ′-size were prepared by changing the primary aging heat treatment temperature. The microstructural evolution was investigated during long term thermal exposure (up to 3000 h) at 900 °C and 1000 °C, respectively. It was found that Cr, Co, and Mo preferentially partition to γ-matrix, while Ta, Ti, Al, Ni, and W partition to the γ′-phase. The abnormal partition of W to the γ′-phase can be attributed to the low content of Ta in CM247 LC alloy. Furthermore, the γ′-coarsening behavior can be divided into two stages. During the early stage of coarsening (<1000 h), the coarsening rate obeys the classical LSW model with the cube rate law. As the increase of the initial γ′-size, the γ′-coarsening rate obviously increases, which can be mainly attributed to the increased solute diffusion capability in the γ-matrix and the higher γ'/γ interfacial energy. By contrast, there is an apparent γ′-coalescence during the later stage of coarsening (1000 h–3000 h). The temperature plays a more dominant role on the γ′-coarsening rate than the duration times during thermal exposure. • The initial-microstructural dependent γ′-coarsening behavior was investigated during long term thermal exposure. • As The increase of initial γ′-size, the coarsening rate constant increases apparently during thermal exposure at 900 °C and 1000 °C, respectively. • The increased γ′-coarsening rate can be ascribed to the increased diffusivity in the matrix and the higher interfacial energy of γ'/γ phases. [ABSTRACT FROM AUTHOR]