Electrical characterization of 60Co γ-ray radiation effects on pentacene-based organic thin-film-transistors having two kinds of gate insulator have been carried out. For transistors with SiO2 gate insulator, the threshold voltage shifts are consistent with positive charge trapping in the oxide and a “rebound” effect is observed. This “rebound” effect is attributed to the negatively charged interface traps generated during irradiation. For polyimide gate insulator, the threshold voltage continually decreases with an increasing total-dose. At total-dose of 1200 Gy (Si), for the SiO2 gate insulator, the field-effect mobility decreased by almost 80%, and for polyimide gate insulator, it decreased by 40%.