The effects of Hf on the precipitation behavior of γ′ and subsequent tensile properties in a Ni-Cr-Mo alloy after long-term thermal exposure are investigated. The results reveal that the addition of Hf leads to an increase in the average diameter of γ′ after thermal exposure at 700℃ up to 5000 h, which promotes the critical resolved shear stress of dislocations shearing γ′ in the Ni-Cr-Mo alloy. Simultaneously, Hf element entering γ′ increases the lattice mismatch between γ′ and γ, enhancing coherency strengthening. These two factors together increase the strength of the alloy. Thus, the yield strength of the Ni-Cr-Mo alloy thermally exposed is effectively enhanced by Hf-alloying at 700℃.