In this work, we investigate the hydroxyl group effect on hysteresis of the low voltage organic thin film transistor (OTFT). A high k material, hafnium oxide, is utilized as gate dielectric to reduce OTFT operational voltage. By using the hydroxyl-free polymer, polystyrene, the hydroxyl groups on hafnium oxide surface will be shielded. The modification at semiconductor/dielectric interface prevents accumulated charges from trapping in surface states. Such a polymer coverage layer reduces hysteresis and suppresses the off current as low as 10(-11) A. The interface traps resulted from ambient water absorption significantly decrease according to the hysteresis cancellation. By stacking polystyrene on hafnium oxide, the pentacene-based OTFT shows the threshold voltage of -2.2 V, on/off current ratio of 10(5), subthreshold swing of 0.34 V/dec, and mobility of 0.24 cm(2)/V s under operational voltage of 10 V. (C) 2010 Elsevier B.V. All rights reserved.