Aligned multiwalled carbon nanotubes have been massively synthesized by the pyrolysis of iron pentacarbonyl (Fe(CO)(5)) and acetylene (C2H2) mixtures in a simply designed horizontal quartz tube reactor. The growth rate and the crystallinity of carbon nanotubes were enhanced by increasing the flow rate of Ar carrier gas. The growth rate, by adopting C2H2 direct bubbling, was dramatically increased compared with Ar direct bubbling, the maximum length of 2000 mum was achieved. (C) 2002 Elsevier Science B.V. All rights reserved.