The dielectric characteristics of poly(ether imide) (PEI)/(Ba0.8Sr0.2)(Ti0.9Zr0.1)O-3 (BSTZ) composites are developed. A circular polarization (CP) antenna with a simple structure is designed on a PEI/BSTZ composite substrate for the applications of ultrahigh frequency (UHF) band radio-frequency identification (RFID) readers. Two rectangular stubs on the proposed circular patch of RFID antennas are used to tune the impedance matching effect and to improve the axial ratio performance. The proposed CP antennas are successfully fabricated on the PEI/BSTZ composite substrates, and an antenna with a similar structure is also fabricated on the FR4 substrate for comparison. In this study, we demonstrate that the CP antenna fabricated on the PEI/BSTZ substrate has better characteristics and is smaller than that fabricated on the FR4 substrate. The antenna fabricated on the PEI/BSTZ composite substrate also has a larger impedance bandwidth and a smaller axial ratio than the antennas fabricated on the FR4 substrate. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3236636] All rights reserved.