We report on single electron transistor (SET) operation in single-walled carbon nanotubes (SWNTs) to estimate the length of ballistic conduction in laser-synthesized SWNTs. The devices were fabricated by an alternating current-aligned method and the SWNTs were side-contacted to the electrodes. At 5 K, Coulomb oscillation and Coulomb diamonds were observed and the Coulomb island length, i.e., the ballistic conduction length, was calculated to be about 200-300 nm. To the best of our knowledge, this is the first report of an SET using an aligned-fabrication methodology and a solution process. (c) 2005 Elsevier B.V. All rights reserved.