In this paper, an experimental methodology for the characterization of the impedance of balanced RF identification (RFID) tag antennas is presented, and the application of the proposed method in RFID tag co-design is demonstrated. The balanced tag antenna is considered as a two-port network and the impedance of the antenna is characterized using network parameters. In the measurement, the antenna is connected to the two ports of a vector network analyzer through a test fixture. The influence of the test fixture is deembedded by using a port-extension technique and the antenna impedance can be extracted directly from the measured S -parameters. The proposed method is useful in practical RFID applications for co-designing the RFID tag with the attached platforms for enhancing the tag performance. An example of co-designing an ultra-high-frequency RFID tag with a plastic Sushi plate is demonstrated. The co-designed tag antenna achieves conjugate matching with the application-specific integrated circuit so that the reading range of the RFID tag is greatly enhanced.