Abstract: The oxidation behaviors of three austenitic cast steels with different morphologies of primary carbides at 950°C in air were investigated using scanning electron microscopy, energy dispersive spectroscopy, and focused ion beam/transmission electron microscopy. Their oxidation kinetics followed a logarithmic law, and the oxidation rate can be significantly decreased as long as a continuous silica layer formed at the scale/substrate interface. When the local Si concentration was inadequate, internal oxidation occurred beneath the oxide scale. The spallation of oxides during cooling can be inhibited with the formation of internal oxidation, owing to the reduced mismatch stress between the oxide scale and the substrate. The “Chinese-script” primary Nb(C,N) was superior to the dispersed primary Nb(C,N) in suppressing the oxidation penetration in the interdendritic region by supplying a high density of quick-diffusion Cr channels. In addition, the innermost and outermost oxidation layers were enriched with Cr, whereas the Cr evaporation in the outermost layer was significant when the water vapor concentration in the environment was high enough. These findings further the understanding regarding the oxidation behavior of austenitic cast steels and will promote the alloy development for exhaust components.