A room-temperature highly-sensitive SO2 sensor with fast response and complete recovery is constructed based on gas dielectric field-effect transistor (FET) of CuPc single crystalline nanowire. The exposed conductive channel by gas dielectric is responsible for the high sensitivity to SO2 and the adsorption of the SO2 molecules dramatically enhances the field-effect mobility. These results not only open up new opportunities to develop new SO2 sensors, but also provide an efficient way to improve the performance of organic FETs.