The reliability of Organic Thin Film Transistors (OTFTs) is of special interest. Here, the instability of the pentacene OTFTs with different gate insulators of SiO2 and polyimide on the measurement ambient pressures has been studied. The gate insulators are thermally grown SiO2 or spin coated polyimide films on heavily doped n(+) silicon wafers. The semiconductor layer of pentacene and the Au layer for the source and drain electrodes were deposited by thermal vacuum deposition. P-channel OTFTs with Si/SiO2(300 nm)/pentacene(70 nm)/Au and Si/polyimide(320 nm-620 nm)/pentacene(70 nm)/Au structures have been fabricated and the transistor electrical characteristics in the air and in the vacuum pressure below 1.3 Pa were measured. As a result, ID-VDS characteristics of the transistor with the gate insulator of SiO2 exhibited notable changes depending on the pressure. On the other hand, the transistor with the polyimide gate insulator exhibited the minimal influence from the ambient pressure. These differences in the ambient pressure dependence are attributed to the difference in the gate insulator/pentacene interfaces.