Organic field-effect transistors (OFETs) were fabricated using poly(3-hexyl thiophene) (P3HT), and the effects of inserted Lewis-acid thin layers on electrical properties were investigated. The OFETs have active layers of P3HT and vanadium pentoxide (V2O5) as a Lewis-acid layer. Larger drain currents were observed for the OFET with the V2O5 layer than that without the layer. The calculated field-effect mobility of the fabricated OFET was 1.4 x 10(-2) cm(2) / (Vs), where as that of the OFET without the V2O5 layer was 6.2 x 10(-3) cm(2) / (Vs). It was thought that charge transfer (CT) complex which was formed at the interface between P3HT and V2O5 layer was dissociated by gate voltage, and the generated holes seem to contribute to drain current and the apparent high mobility.