High purity carbon nanotubes are synthesized by methane decomposition using an arc-jet plasma of high temperature (5000-20, 000 K). Since the arc-jet plasma process is continuous and easily scalable, it is a promising technique for the large-scale commercial production of carbon nanotubes. In this experimental work, the are-jet plasma is generated by a dc non-transferred plasma torch, in which a mixture of argon and hydrogen is used as a plasma forming gas and nickel powder as a metal precursor. Morphology, crystallization degree and purity of the carbon nanotubes in the soot produced under various processing conditions Eire evaluated by using scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and thermogravimetric analysis. From the results of these material analyses, we have found that multi-walled carbon nanotubes of high purity were produced in the optimal operating condition. In addition, the thermal plasma characteristics for the carbon nanotubes growth are discussed from numerical simulation result of the arc-jet plasma. (c) 2005 Elsevier B.V. All rights reserved.