Organic field-effect transistors (OFETs) with Lewis-acid films were fabricated using copper phthalocyanine (CuPc), and the operating mechanism was studied. These OfETs have active layers of CuPc and vanadium pentoxide (V2O5) as a Lewis-acid film. A large drain current was observed in OFET's with a V2O5 layer for a positive gate voltage, and the mobility of these OFETs was improved compared with OFETs with only CuPc as an active layer. To investigate the operating mechanism, Xray diffraction patterns, UV-vis absorption spectra, and ionization potentials were measured in V2O5 films and CuPc films with and without a co-evaporation Of V2O5. Each characteristic of the CuPc layer changed owing to the introduction Of V2O5 It was considered that a CT complex is formed in the CuPc layer owing to the interaction between CuPc and V2O5 molecules, and contributes to the large drain current and improvement in mobility.