[en] Highlights: • Influences of Mo/Zr minor-alloying on the phase precipitation in modified 310S ASSs at high temperatures are investigated. • The excess Mo could accelerate the phase transformation of Cr₂₃C₆ to σ after long-time aging at 973 K. • The excess Zr and a high Zr/C ratio would lead to the σ precipitation at the early stabilization stage. • The Fe-22Ni-25Cr-0.046C-0.37Mo-0.35Zr alloy exhibits the best microstructural stability and good mechanical property. • The impurity Si should be controlled strictly to inhibit Ni₁₆Si₇Zr₆ phase precipitation in ASSs containing strong carbide-forming elements. High-Cr/Ni austenitic stainless steels (ASSs) have attracted more attention as fuel cladding materials of super-critical water reactors due to their excellent comprehensive properties. In order to further improve their microstructural stability at high temperatures, the present work investigated systematically the influences of Mo/Zr contents and Zr/C ratios on the phase precipitation behaviors and mechanical properties of modified 310S ASSs. The designed alloy ingots were hot-rolled, solid-solutioned at 1423 K for 0.5 h, stabilized at 1173 K for 0.5 h, and then aged at 973 K for different hours. The microstructure and precipitated phases at different heat-treatment states were characterized with OM, SEM, EPMA and TEM, respectively. All the results indicated that the excess addition of Mo and Zr and the inappropriate Zr/C ratios would promote the formation of Cr₂₃C₆, G-Ni₁₆Si₇Zr₆ and (Ni,Fe)₂₃Zr₆ phases, resulting in the σ phase precipitation at the early stabilization stage. Furthermore, the formation mechanism of σ phase was discussed. The effects of the precipitated phases on the mechanical properties of alloys were then studied. It was found that the Fe-22Ni-25Cr-0.046C-0.37Mo-0.35Zr (wt%) alloy with appropriate Mo content and Zr/C ratio of 1/1 exhibits the best microstructural stability and good tensile mechanical property, in which only a few σ particles are precipitated from the matrix even after aging at 973 K for 408 h.