A square patch passive RFID tag antenna designed for UHF band is presented in this article. To achieve compact size and circular polarization (CP) radiation, the square patch is embedded with a cross slot, while an L-shaped open-end microstrip line linked to a tag-chip and terminated by a shorting pin is capacitively coupled to the patch. By selecting an appropriate length for the microstrip line and its coupling distance with the radiating element, easy control on the input impedance of the proposed tag antenna which leads to excellent impedance matching is achieved. The measured 10-dB return-loss bandwidth of the tag antenna is 50 MHz (904-954 MHz), while its 3-dB axial-ratio bandwidth is 6 MHz (922-928 MHz). Further experiment shows that the tag antenna can provide better reading range when mounted on a metallic surface.