The dependence of carbon nanotube diameters upon the size of nickel catalyst particles supported on amorphous carbon films was studied. Nanotubes were catalytically grown at different temperatures to elucidate the effect of temperature. The transformation of nanotube-growth mechanism takes place in the range 700-800 degreesC as evident from particle size-nanotube diameter relations, tip particles and nanotube morphologies. At low temperature (700 degreesC), the nanotube growth is conducted through solid tip catalyst particles. At 800 degreesC, nanotubes grow via liquid catalyst particles by extrusion mode. Low-temperature tube diameters reproduce essential features of original particle size distribution. In contrast, high-temperature tubes exhibit universal Gauss-like diameter distribution irrespective of catalyst particle sizes. (C) 2002 Elsevier Science B.V. All rights reserved.