The effect of La2O3 on the oxidation resistance of SiC ceramic at 1973 K was investigated using isothermal oxidation test and first-principles calculations. The SiC ceramic with La2O3 shows a better oxidation resistance compared with that without La2O3 due to the in situ formed La2Si2O7 in SiO2 glass layer after oxidation at 1973 K. First-principles calculations based on density functional theory were applied to analyze the solution behaviors of La atom in the surface of SiO2 and La2Si2O7. The solution energy of La atom in SiO2 (0 1 1) is -19.05 eV, which is far less than -4.19 eV in La2Si2O7 (2 0 1) with a La vacancy, thus resulting in that La atom in La2Si2O7 (2 0 1) diffuses into SiO2 (0 1 1). The SiO2 lattice with an interstitial La atom is more stable than that with a substitutional La atom and the interstitial La breaks the nearest Si-O bond to form La-O and La-Si bonds, which is beneficial to improving the high-temperature stability of SiO2. Experimental and theoretical results indicate that the formation of refractory La2Si2O7 phase enhances the stability of SiO2 glass layer, so as to protect SiC ceramic from further oxidation at 1973 K.