Si₃N₄–SiC ceramics were prepared via reaction sintering with SiC, silicon powder and SiO₂ as main raw materials, and the ceramics were oxidized at a high temperature. The structure and properties of ceramics before and after high-temperature oxidation were investigated. The results show that the main crystalline phases of the samples before high-temperature oxidation are SiC, α-Si₃N₄/β-Si₃N₄ and a small amount of silicon powder, Si₂N₂O and triclinic-SiO₂, while the silicon powder disappears after high-temperature oxidation and forms tetragonal-SiO₂. The mass fraction of β-Si₃N₄ remains constant, and the mass fraction of α-Si₃N₄ decreases from 11.3% before high-temperature oxidation to 6.8% after high-temperature oxidation, while the mass fraction of Si₂N₂O and SiO₂ increases, indicating that α-Si₃N₄ can be decomposed into Si₂N₂O and SiO₂ more easily than β-Si₃N₄. After high-temperature oxidation, the flexural strength of samples decreases from (68.55±6.36) MPa to (49.80±4.96) MPa mainly due to the decomposition of α-Si₃N₄ into Si₂N₂O and SiO₂.