We report the effects of moisture on the electrical characteristics of pentacene field-effect transistors (FETs) with the polyvinylpyrrolidone (PVPy) gate insulator. For the condition of relative humidity below 40%, the pentacene FET exhibited a stable operation without a shift in the threshold voltage upon a gate voltage sweep direction. With increasing the relative humidity above 50%, the threshold voltage shifted critically toward the positive direction, accompanied by significant degradations in the field-effect mobility and the subthreshold slope. These moisture-induced characteristic degradations could be substantially recovered by the low-temperature annealing process under a base pressure of 2 x 10(-3) Torr.