The synthesis of Ni-catalyzed multiwalled carbon nanotubes and nanofibers on a catalytic metal substrate, using an ethylene-fueled inverse diffusion flame as a heat source, was investigated. When the approximate gas temperature was varied from 1400 to 900 K, carbon nanotubes with diameters of 20-60 nm were formed on the substrate. In the regions where the approximate gas temperature was higher than 1400 K or lower than 900 K, iron nanorods or carbon nanofibers were synthesized, respectively. Based on the quantitative analyses of many scanning electron microscope (SEM) and transmission electron microscope (TEM) images, the nanotubes formed closer to the flame had a tendency to have larger diameters. High-resolution TEM images and Raman spectra revealed that the synthesized carbon nanotubes had multiwalled structures, with some defective graphite layers at the wall. Based on the graphite mode of the Raman spectra, it was believed that the optimal synthesis could be obtained if the substrate was positioned at between 5.5 and 5.0 mm from the flame center. However, in the case of higher gas temperatures with respect to the formation of nanotubes rather than nanofibers, the graphite layers on the catalyst metal might be under relatively strong stress due to the large amount of precipitated carbon atoms on the edge. As a result of this strong compressive stress the carbon nanotubes are formed with empty or periodic compartments which are hollow on the inside. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.