N-channel organic field effect transistors (FETs) with F16CuPc/SiO2(300nm)/Si and F16CuPc(20nm)/Polyimide(320nm)/Si structures were fabricated, and the transistor electrical characteristics measured in the air and in a vacuum at a pressure below 1.3Pa were compared. The threshold voltage V-th and the mobility of the transistor with the SiO2 gate insulator are strongly dependent of the ambient pressure during the measurements, while those of the transistor with the Polyimide gate insulator are nearly independent of the ambient pressure during the electrical measurements. The change of the transistor characteristics on the ambient pressure might be attributed to the absorption and desorption of some negatively charged chemical species on the transistor channel interface region.