O-2(+) ion beams are used to modify the surface of 50 nm thick Fe catalyst films deposited on Si(100) substrates to demonstrate selective area growth of aligned multiwall carbon nanotubes (MWNT). Aligned MWNTs were grown on the modified Fe/Si substrate using the hot filament plasma enhanced chemical vapor deposition (HF-PECVD) method. A higher growth rate and density of MWNTs was observed on the ion-modified areas, facilitating selective area growth of aligned MWNTs at temperatures as low as 560 degreesC. Deposition of graphitic sheets at the initial growth process on the unmodified areas inhibits the deposition of MWNTs. (C) 2003 Elsevier Science B.V. All rights reserved.