A planar I-shaped folded-patch antenna with a footprint of 21 mm × 21 mm × 1.6 mm is designed for compact UHF RFID tags to cohere on metal. The antenna consists of three parts: a square ground plane, an I-shaped patch, and a ring resonator. The I-shaped patch is interconnected to the ground plane through a narrow shorting stub, and the microstrip feed line is inserted into the patch to reduce the input impedance of the patch. Extra capacitance and inductance introduced by the ring resonator can lower the tag's resonant frequency down to the expected UHF RFID band. The proposed antenna is manufactured, and there is excellent consistency between simulation and measurement results. The proposed tag antenna achieves a far read distance up to 6.3 m on metal (with 4 W equivalent isotropic radiated power) at resonant frequency of 920 MHz.