We have studied the growth of carbon nanotubes by chemical vapor deposition (CVD) on the nanoparticles of iron oxide with two different diameters, 4 and 10 nm, supported on MgO powder. Both nanoparticles catalyzed the growth of double-wall carbon nanotubes (DWNTs) together with a small amount of single-wall carbon nanotubes (SVTNTs). We found that the inner- and outer-diameters of the DWNTs were modified by the initial nanoparticle size; smaller nanoparticles catalyzed the growth of the DWNTs with smaller diameters. These results suggest that the MgO support strongly influence the catalytic activity of nanoparticles by reduction of the catalytically-active size. (C) 2004 Elsevier B.V. All rights reserved.