Highly graphitized multi-walled carbon nanotubes (MWNTs) were mass-produced by hydrogen arc discharge with automatic feeding of carbon electrode. Refluxing them first in hydrogen peroxide and then in a mixture of sulfuric and nitric acids was carried out for purification of MWNTs. The purification process of MWNTs was evaluated by scanning electron microscopy. Nanostructure and crystalline perfection of the purified MWNTs were investigated by using high-resolution transmission electron microscopy and Raman spectroscopy. It was found that the purified MWNTs had about 1.3 nm innermost diameter, resulting in the appearance of radial breathing modes in their Raman spectra. (C) 2002 Elsevier Scicnce B.V. All rights reserved.