Zirconium diboride based composites containing silicon carbide with relative densities in excess of 99 % were produced by hot-pressing. Oxidation test was conducted in air at 1500 °C. ZrB2-SiC composite showed relatively low oxidation resistance due to the non-uniform surface silica-rich layer. But in case of mixed boride-SiC composites further improvement of the oxidation performance were observed due to the phase separation in the surface silica-rich layer.