A polymer FET device was fabricated with poly(3-hexylthophene) (P3HT) as a active layer and PMMA/PVA bilayer as a gate dielectric. The influence of the measurement atmosphere on the FET property was investigated. The offcurrent was increased and the threshold voltage (V-th) shifted positively by exposing the device in the air. Investigation of the I-G-V-G behavior and electro-absorption spectra indicated that the observed phenomena were mainly due to the presence of the excess carrier in the active layer which was produced by the oxidation of P3HT. The shift of V-th under the air is mainly due to the decrease of the redox potential of P3HT.