Single-walled carbon nanotubes (SWWs) have been synthesized using the alcohol chemical vapor deposition technique. On comparing the SWNTs synthesized using different catalyst compositions, an obvious fact has been clarified - the diameter of SWNTs is strongly dependent on the sizes of the catalyst particles and metal species. Iron atoms exhibit a strong interaction with carbon atoms and keep their diameters narrow. Cobalt atoms exhibit a poorer interaction than the iron atoms, but work to decompose ethanol. As a result, narrow SWNTs are generated on the iron-rich catalyst. In this Letter, we propose the most probable growth mechanism of the SWNTs. (c) 2005 Elsevier B.V. All rights reserved.