hin film organic transistors with Si/SiO2 (300 nm)/pentacene (70 nm)/Au and Si/polyimide (320-690 nm)/pentacene (70 nm)/Au structures have been fabricated and their performances have been compared. The evaluated mobility of the holes of the SiO2 transistor was 0.002 cm(2)/Vs, while those of the polyimide transistors were between 0.026-0.031 cm(2)/Vs. On the other hand, the threshold voltage of the SiO2 transistor was -15 V, while the threshold voltages of the polyimide transistors were approximately +4V independent of the polyimide thickness. This apparently strange behavior of the polyimide transistors might be attributed to some more negatively charged species accumulating at the polyimide/pentacene interface as the thickness of the polyimide film increases. The PI films had a significantly low level of the leakage current, and the estimated gate-drain leakage current of the corresponding transistor at the gate voltage of -100V is estimated to be negligibly small compared with the source-drain current.