Monodispersed multi-walled carbon nanotubes were prepared in a chemical vapor deposition (CVD) method by using monodispersed nanoparticles of Fe3O4 (1 nm) as the precursor of catalyst particles. High-resolution electron microscopy (HREM) revealed that over 50% of these carbon nanotubes synthesized at 600 degreesC and 180 Torr were quadruple walled nanotubes with mean outer diameter of 5.6 nm. At the same time, in the close tips of these carbon nanotubes, no catalyst particles were detected by HRTEM, which means that the catalyst particles were too small to be detected by HRTEM, even though they exit in the tips of carbon nanotubes in our experiments. (C) 2002 Published by Elsevier Science B.V.