Electric field assisted assembly of single walled carbon nanotubes (SWCNTs) on lithographically patterned electrodes has been studied using both dc and ac electric fields. The nanotube alignment is strongly dependent on the magnitude and the frequency of the applied electric field. An improved carbon nanotube (CNT) orientation is achieved with ac electric field of high frequency due to alternating force exerted rapidly on field-induced dipoles of the nanotubes and the nanotubes orient nearly at right angle to the metal electrodes. The purification of as-prepared CNTs from background nanoparticles appears possible by aligning nanotubes with an ac electric field. (C) 2003 Elsevier B.V. All rights reserved.