GH984G alloy is a precipitate-strengthened Ni–Fe based alloy considered as boiler materials in next generation of advanced ultra-supercritical (A-USC) coal fired power plant. In this paper, the effect of Nb/(Ti + Al) ratio on the phase stability and tensile properties of GH984G alloy was investigated. Detailed investigations reveal that the major precipitates of the alloys with different Nb/(Ti + Al) ratios are all γ', MC and M23C6. The higher Nb/(Ti + Al) ratio retards γ′ nucleation. During thermal exposure, γ' coarsening behavior of the alloys with different Nb/(Ti + Al) ratios all follows Ostwald ripening kinetics. However, the higher Nb/(Ti + Al) ratio accelerates γ' coarsening at early stage, but its influence decreases with increasing thermal exposure time. The reasons are attributed to the influence of Nb/(Ti + Al) ratio on γ/γ' misfit and the diffusion behavior of γ' forming elements. The thermal stability of MC carbide and the precipitation and growth of M23C6 carbide are no dependence on Nb/(Ti + Al) ratio. During thermal exposure, the higher Nb/(Ti + Al) ratio has a negative effect on yield strength, but its influence decreases with increasing thermal exposure time. The reasons are attributed to the influence of Nb/(Ti + Al) ratio on the precipitation behavior of γ′ phase. In addition, Nb/(Ti + Al) ratio has no notable influence on ductility and fracture behavior.