To see roles of an ambient gas on the formation process of single-wall carbon nanotubes (SVVNTs), we have prepared SWNTs by laser ablation with Ne, Ar, Kr and N-2 gases and estimated the abundances of SWNTs mainly by Raman spectroscopy. The abundance of SWNTs was found to depend systematically on the pressure and the molecular mass of the ambient gas, and the highest abundance was obtained in N-2 at 1000 Torr. Only N-2 produced thinner SWNTs than any other gases, which suggests that the size of nanotube precursors is sensitively affected by the internal freedom of ambient gas. (C) 2003 Elsevier Science B.V. All rights reserved.