We have studied the current-voltage characteristics and threshold voltage behaviors in polycrystalline pentacene-based organic thin-film transistors (OTFTs) with gold top-contact electrodes for different thickness of the pentacene films. The study uses a number of techniques to measure threshold voltage including: square-root method, constant-current method, maximum transconductance method and capacitance-voltage measurements. The results of our experiments suggest that the space-charge and trapping effects of the pentacene bulk film plays an important role in current-voltage characteristics. In the presence of trapping centers within the polycrystalline pentacene film, we have provided an analytical model explaining the effect of bulk traps on the thickness-dependent threshold voltage. Square-law of threshold voltage versus pentacene thickness in OTFTs was derived during our study providing consistent results with experimental data. Furthermore, using the square-law, we estimated trap density of about 10(17) cm(-3) in experimental pentacene films. The orders of magnitude in trap density of polycrystalline pentacene films are similar to the values found in other literature source. (C) 2009 Elsevier Ltd. All rights reserved.