In this article, a compact RFID tag antenna operated on UHF band with circular polarization is proposed. The antenna is comprised of a radiating square with two pairs of unequal crossed slots to produce a circularly polarized (CP) radiation and a simple quarter-wave microstrip line to feed the patch. A tag chip is embedded between the microstrip line and a short- circuited stub line. The simulated result for −3-dB power reflection coefficient (PRC) is about 27 MHz (904–931 MHz) that covers the RFID bands of North and South America and Taiwan (920–928 MHz) and its corresponding simulated 3-dB axial-ratio (AR) bandwidth is about 7 MHz (910–917 MHz). The overall antenna size is 75 mm × 75 mm × 1.6 mm at 915 MHz. The results show a perfect circularly polarized radiation.