Carrier injection, carrier transport and carrier accumulation are three key process to understand the characteristics of organic field effect transistor (OFET) devices. In our previous studies, we showed the evidence of carrier injection from source electrode by means of optical second harmonic generation (SHG) measurement and capacitance-voltage (C-V) measurements, and explained the FET characteristics using a Maxwell-Wagner model. In this paper, to further clarify the behavior of the carrier transport and hole injection from source electrode, we focused on the hysteresis behavior observed in the current-voltage (I-V) and C-V characteristics of pentacene FETs. Employing the electric field induced SHG (EFISHG) and C-V measurements, we could show that the origin of the hysteresis behavior is caused by holes, which are injected and subsequently trapped in FET channel. (c) 2006 Elsevier B.V. All rights reserved.