The growth behavior of carbon nanotubes synthesized from C2H2 using thermal chemical vapor deposition method has been investigated. The formation of catalytic cobalt nanoparticles was much enhanced when using NH3 as the environment gas, whereas NH3 introduced during nanotubes growth does not play a key role in growing the vertically aligned nanotubes. The role of NH3 is to obtain the high density of nucleation-sites for nanotube growth by inhibiting amorphous carbon generation in the initial stage of synthesis. We found unexpected behavior in terms of the length and growth time during nanotube growth. The length of the nanotubes is inversely proportional to the size of nanoparticle just in the initial stages of growth and thick catalyst. (C) 2003 Elsevier Science B.V. All rights reserved.