Carbon nanotubes (CNTs) were grown from Co nanoparticles by thermal chemical vapor deposition using C2H2, H-2, and Ar gases. The diameters of CNTs were limited by the sizes of the catalytic Co nanoparticles. From 5 nm Co nanoparticles, very long and straight CNTs with diameters below 5 nm were grown. C2H2 was so effective as the carbon feedstock that the required amount of C2H2 was only 0.1% of the total amount of gases to grow CNTs from 5 nm Co nanoparticles. For 8 nm Co nanoparticles, a long enough pretreatment time was essential in growing dense CNTs. (C) 2003 Elsevier Science B.V. All rights reserved.