Double wall carbon nanotubes (DWCNTs) were mass-produced by pyrolizing C2H2 at the temperature range of 900-1100 degreesC on a floating iron catalyst with sulfur promoted. Our experiments indicated that the growth of DWCNTs was strongly dependent on the sulfur addition, and without sulfur only single wall carbon nanotubes (SWCNTs) were produced. The outer tube diameters of DWCNTs observed by high resolution electron microscopy (HRTEM) vary from 1.05 to 2.89 nm, and the inner tube diameters vary from 0.40 to 2.19 nm. The effect mechanism of sulfur to the growth of DWCNTs was discussed. (C) 2002 Published by Elsevier Science B.V.