A gas-to-particle conversion mechanism is proposed for producing silicon carbide ultrafine particles by a SiH4/C2H2 chemical vapor deposition reaction system. The reaction was performed in a horizontal hot-wall tubular reactor at a temperature range of 1123-1373 K, where SiC was formed as an aerosol in the gas phase and deposited to form porous SiC films. An analysis of the deposition growth rate profile along the longitudinal direction of the reactor shows that the depositing process is controlled by a gas-phase reaction with an activation of 23 kcal/mol. Also, the composition ratio of carbon to silicon of the deposited films was found to change gradually from 1 at the inlet of the reactor to 2 downstream. The results imply that a gaseous polymerization reaction between C2H2 and SiH2 plays an important role in the formation of the particles.