This invention is about Organic field effect transistor CO sensor and the method of making the same. It is a bottom gate bottom contact organic field effect transistor (OFET) CO sensor comprising a gate electrode, a gate dielelectric layer formed on the gate electrode, source and drain electrodes formed on the gate dielectric layer and a CO sensing organic layer formed on the source and drain electrodes. The organic layer comprises a blend of Fe (III) porphyrin and a compatible solution processed organic semiconductive material. The method of making (OFET is comprised of forming a dielectric layer on a gate electrode; forming source and drain electrodes on the gate dielectric layer and forming a CO sensing organic layer on the source and drain electrodes. This organic layer is formed with a blend of Fe(III) porphyrin and a compatible organic semiconductive material in solution with an organic solvent and is annealed. Besides being CO detectors with high selectivity and sensitivity and fast response to CO, the CO sensors of the invention also have various other advantages like high flexibility and versatility.