A new type of component, the patch antenna with the relative humidity sensing function using a modified polyimide is proposed for RFID. The proposed structure can be applied for RFID sensor tag as a physically and functionally combined antenna with the RH sensor. For high sensitivity, a polyimide film was synthesized from aromatic monomers where no hydrophobic element is included. The fabricated patch antenna with RH sensing function was characterized in view point of sensitivity. Furthermore, to enhance the resolution of the sensing function, the patch antenna was designed to have a narrower bandwidth by meandering structure. From the measured results, the sensitivity of the proposed structure was -108 kHz/%RH under 25%RH - 90%RH. This means that the proposed component provides the humidity sensing function as well as the radiation, Therefore, it is apparent that the proposed RH sensing antenna using a modified polyimide can be applicable and useful for the implementation of the compact RFID sensor tag.