Silicon carbide nanowires (SiCNWs) are a set of promising reinforcement materials due to their superior properties. However, formation mechanism of the SiCNWs synthesized by the thermal evaporation method without metal catalyst is still unclear. To understand the formation mechanism, SiCNWs were synthesized by the thermal evaporation method at 1350 • C using a pre-oxidized Si powder and CH 4 gas as precursors. SiCNWs obtained by this method were ␤-SiC/SiO 2 core-shell nanowires with average diameter about 55 nm and with a length up to 1 mm. The exhaust gases during the SiCNWs synthesis process were examined by gas chromatography and the photographs of growth activity of SiCNWs inside the furnace were captured. CO gas was detected during the active formation of SiCNWs. It was clarified that CO gas was one of the byproducts from SiCNWs synthesis process, and the formation reaction of SiCNWs should be 3SiO(g) + 3C(s) → 2SiC(s) + SiO 2 (s) + CO(g). The formation of SiCNWs was discussed based on the oxide-assisted-growth mechanism.