The effect of NH3 plasma pre-treatment on the growth characteristics of CNTs was investigated. We observed that NH3 plasma pre-treatment etched and conglomerated Ni catalyst film, resulting in the formation of Ni nanoparticles. The aligned CNTs from the Ni nanoparticles were grown by plasma enhanced chemical vapor deposition (PECVD). As Ni film thickness decreased from 300 to 30 Angstrom, the size of Ni nanoparticles decreased about from 140 to 90 nm and the average diameter of CNTs became smaller. As the NH3 plasma power was increased, the density of Ni nanoparticles was decreased, leading to the decrease in the density of CNTs. (C) 2003 Elsevier Science B.V. All rights reserved.