Abstract: Double-layer SiC-based coatings on carbon/carbon composites were prepared by a wrapping cementation method with different oxides (MgO, Al₂O₃ or B₂O₃) as additives to the inner layer. The phase morphologies and antioxidation properties of the SiC coated carbon/carbon composites were investigated by SEM, EDS and XRD. Results show that the SiC coating with B₂O₃  as an additive to the inner layer is more compact than those with MgO or Al₂O₃ as the additive. Different phases exist in the three different inner layer SiC coatings. The double-layer SiC coating modified by B₂O₃ is dense and uniform with few flaws and holes, and can protect carbon/carbon composites from oxidation at 1 500℃ for 200 h in air.