Randomly oriented carbon nanotubes (CNTs) were synthesized directly on SiO2 substrates by thermal chemical vapor deposition (CVD) with C2H4 gas as a carbon source. To control the diameters of the carbon nanotubes, Fe-Ni thin films with different thickness from 80 to 320 nm were deposited as catalysts on SiO2 substrates by magnetron sputtering. The diameter of the resultant carbon nanotubes was dependent on the size of the catalyst particles. Moreover, a small catalyst particle permitted carbon nanotube growth. The resultant randomly oriented carbon nanotubes exhibited good field emission proper-ties. (C) 2004 Elsevier B.V. All rights reserved.