A new kind of sandwich-like bis[2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato] Prascodymium complex Pr(Pc-*)(2) [Pc-*=Pc(OC8H17)(8)] is used as film-forming material. Pure Pi-(Pc-*)(2) ultrathin film and Pr(Pc-*)(2) and octadecanol(OA) mixed ultrathin films were prepared by the Langmuir-Blodgett (LB) technique. Based on the conventional metal-oxide-semiconductor-field-effect transistor (MOSFET), a new chemical field-effect transistor (ChemFET) gas-sensing device was fabricated by depositing the Pr(Pc-*)(2)/OA (1:3 molar ratio) mixed LB film on the gate area of MOSFET replacing the gate metal. e sensitive property of Pr(Pc*)2/OA mixed LB film based ChemFET sensor to nitrogen dioxide (NO2) gas is studied by the change of drain current (I-ds) during gas exposure. The Pr(Pc*)(2)/OA mixed LB film ChemFET gas sensor can detect NO2 gas down to 5 ppm. The qualities are attributed to amplification by the FET and the ordered nature of LB film. The mechanism of sensitivity of Pr(Pc*)(2)/OA mixed LB film ChemFET to NO2 is also studied in the paper.