We studied the channel region at the gate insulator-active layer interface in pentacene field effect transistors (FETs) by using the electric field induced optical second harmonic generation (EFISHG) measurements. The SH signal was enhanced along the interface, dependent on biasing conditions, and reflected the region of accumulated holes that were injected from the source electrode. Analysing the charge accumulation condition along the channel, we explained the experimental EFISHG results. (C) 2009 Elsevier B. V. All rights reserved.