We report on the insulator-charging-effects of poly-4-vinylphenol (PVP) gate dielectric on the reliabilities of pentacene thin-film transistors (TFTs). Our PVP films were prepared by spin coating and curing at various temperatures (155, 175, and 200 °C). Evaluated using Au/PVP/p
+−Si structures, the dielectric strength of PVP films cured at 175 °C was superior to those of the other PVP films cured at different temperatures. Although saturation current and field mobility (∼0.13 cm2/Vs) obtained from a TFT with PVP film cured at 200 °C appeared higher than those (∼0.07 cm2/Vs) from the device with 175 °C-cured polymer film, the TFT prepared at 200 °C revealed a low on/off current ratio of less than 104 due to its high off-state current and also unreliable saturation behavior under repetitive gate voltage sweep. The unreliable behavior is due to the dielectric-charging caused by gate-electron-injection. We thus conclude that there are some optimal PVP-curing conditions to improve the reliability of pentacene TFT.