The corrosion and mechanical properties of austenitic stainless steels can be enhanced considerably by adding Nb. Newly developed Nb-stabilized austenitic stainless steels, such as 347HFG, 316Nb, TP310HCb, NF709, and HT-UPS, exemplify this advancement. The required Nb content varies across these steels. Prior research has indicated that in the as-cast microstructure of these steels, coarse and unevenly distributed primary NbC often forms, adversely affecting their mechanical and corrosion properties. Furthermore, this coarse primary NbC depletes the solid solution of Nb, which is counterproductive for fine secondary NbC precipitation. Notably, modifying the morphology and size of primary NbC through hot working and heat treatment is challenging. To enhance the microstructure and mechanical properties of Nb-stabilized austenitic stainless steel, this study investigated the effects of Nb content and homogenization treatment on these steels. The microstructure and tensile properties of cast austenitic stainless steel were analyzed using OM, SEM, TEM, and tensile test. The findings reveal that varying Nb content influences the precipitation of primary NbC and M₂₃C₆ carbides. In Nb-free steel, M₂₃C₆ carbides precipitate continuously at grain boundaries. This precipitation still occurs in steel with 0.30%Nb (mass fraction), alongside the formation of NbC + γ eutectic structures. Increasing Nb content to 0.90% can suppress M₂₃C₆ carbide precipitation, although the eutectic structures become more prevalent. A notable enhancement in yield strength accompanies an increase in Nb content to 0.90%. This improvement is attributed to the solid solution strengthening by Cr (due to suppressed M₂₃C₆ carbides) and Nb, grain boundary strengthening from refined grain sizes, and precipitation strengthening by secondary NbC. However, microcracks are easily nucleated at primary NbC/γ interface under plastic deformation, leading to rapid crack propagation along primary NbC networks and resulting in trench-like brittle fractures. This mechanism significantly reduces elongation. Post-homogenization treatment at 1250 ^oC alters the primary NbC morphology from rod-like to spherical/ellipsoid. This change increases the critical stress required for microcrack nucleation at NbC/γ interfaces, thereby inhibiting microcrack initiation. Additionally, the primary NbC networks transform from continuous to discontinuous distributions, impeding microcrack propagation. Consequently, this treatment significantly enhances elongation without compromising strength.