By using time-resolved microscopic optical second-harmonic generation (TRM-SHG) imaging, we studied the trap-filling effect on transient carrier transport in pentacene field effect transistors (FETs). We showed that the transient carrier transport was strongly dependent on trap-filling condition at the pentacene and gate-insulator interface. The TRM-SHG imaging caught the transient electric field migration which was caused from traveling carriers along the channel. Results showed that the effective carrier mobility increased with increase of the filled-trap density, i,e., with decrease of empty trap-sites, and finally saturated after all trap-sites were filled. Saturation voltage of the mobility indicates the filled trap density of approximately 1.4 x 10(12)/cm(2). (C) 2011 Elsevier B.V. All rights reserved.