A novel oxidation resistant SiC/B4C/C nanocomposite was prepared by the pyrolysis of carborane-containing poly(silyleneethynylenephenyleneethynylene) (CB-PSEPE) at 1450°C in argon. The phase composition and microstructure of the carbon-rich ceramic were investigated by X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The β-SiC (~12 nm) and B4C (~20 nm) nanocrystalline phases were formed during the pyrolysis process of CB-PSEPE. The presence of boron in CB-PSEPE promotes the growth of β-SiC nanocrystals. The ceramic shows excellent thermooxidative stability based on thermogravimetric analysis in air. A borosilicate layer was formed on the surface of the ceramic after oxidation. This carborane-containing conjugated polycarbosilane could be a potential precursor for oxidation resistant carbon-based materials.