The development of Gen-IV nuclear systems and ultra-supercritical power plants proposes greater demands on structural materials used for key components. An Fe–18Ni–16Cr–4Al(316-base) alumina-forming austenitic steel was developed in our laboratory. Its microstructural evolution and mechanical properties during aging at 950°C were investigated subsequently. Microstructural changes were characterized by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Needle-shaped NiA l particles begin to precipitate in austenite after ageing for 10 h, whereas round NiA l particles in ferrite are coarsened during aging. Precipitates of NiA l with different shapes in different matrices result from differences in lattice misfits. The tensile plasticity increases by 32.4% after aging because of the improvement in the percentage of coincidence site lattice grain boundaries, whereas the tensile strength remains relatively high at approximately 790 MPa. The development of Gen-IV nuclear systems and ultra-supercritical power plants with greater demands on structural materials used for key components. An Fe-18Ni-16Cr-4Al (316-base) alumina-forming austenitic steel was developed in our laboratory. Its microstructural evolution and mechanical properties during aging at 950 ° C were investigated subsequently. Microstructural changes were characterized by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Needle-shaped NiAl particles begin to precipitate in austenite after aging for 10 h, Precipitates of NiA l with different shapes in different matrices result from differences in lattice misfits. The tensile plasticity increases by 32.4% after aging because of the improvement in the percentage of coincidence site lattice grain but the tensile strength remains relatively high at about 790 M Pa.