A compact circularly polarized circular microstrip tag antenna designed for RFID UHF band is presented. A cross slot of unequal arm lengths embedded in a circular patch is used to achieve compact CP operation. By choosing the proper lengths of two short-circuited arc microstrip lines and its coupling distance, good complex impedance matching can be obtained. The measured 10-dB return-loss bandwidth of the tag antenna is 40 MHz (901–941 MHz), while its 3-dB axial-ratio bandwidth is 6 MHz (912–918 MHz). Further experiment shows that the tag antenna can provide better reading range when mounted on a metallic surface.