Double-walled carbon nanotubes (DWNTs) were synthesized from coal in large quantity by arc-discharge method in hydrogen-free atmosphere, which were systematically examined using scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. The results show that the as-synthesized DWNTs have an outer diameter of 1.0-5.0 nm with an interlayer spacing in the tube walls of ca. 0.41 nm. The possible mechanism involved in the formation process of DWNTs is proposed and discussed in term of the unique chemical composition of coal and the process parameters adopted in the study. (c) 2006 Elsevier Ltd. All rights reserved.