Metal nanoparticles play an important role in chemical vapor deposition (CVD) synthesis of carbon nanotubes because nanoparticles not only catalyze the nanotube growth but also determine the structural characteristics of the nanotubes. We report on the synthesis of single-wall carbon nanotubes (SWNTs) on the basis of the gas-phase reaction of colloidal solutions of metal nanoparticles containing Co and Mo. The colloidal solution of the nanoparticles is prepared by a reverse micelle method and injected into a furnace, where the solvent serves as the carbon source while the nanoparticles act as the catalyst. We have found that addition of a small amount of thiophene leads to formation of the SWNTs. The formation mechanism of the SWNTs is discussed by comparing the present CVD and laser-ablation methods.