A slim radio frequency identification (RFID) tag antenna design for metallic objects application is proposed in this letter. It is designed based on a high-impedance surface (HIS) unit cell structure directly rather than adopting a large HIS ground plane. The antenna structure consists of metallic rectangular patches electrically connected through vias to the ground plane to form an RFID tag antenna that is suitable for mounting on metallic objects. The experimental tests show that the maximum read range of RFID tag placed on a metallic object is about 3.1 m and the overall size is 65 times 20 times 1.5 mm3. It is thinner than inverted-F antenna (IFA), planar inverted-F antenna (PIFA), or patch-type antennas for metallic objects. Simulation and measurement results of the proposed RFID tag antenna are also presented in this letter.