Polyetherimide/(Ba0.8Sr0.2)(Ti0.9Zr0.1)O3 (PEI/BSTZ8291) composites are fabricated using PEI, dispersant, solvents, and BSTZ powder. The effects of the content of BSTZ8291 filler on the chemical, physical, mechanical and dielectric properties of PEI/BSTZ8291 composites are studied in this paper. As the content of BSTZ filler increases from 10 wt% to 70 wt%, the relative permittivity of PEI/BSTZ8291 composites at 1 MHz increase from 2.58 to 17.71. The measurement of relative permittivity of PEI/BSTZ8291 composites is developed using the ¡§Rectangular Cavity Resonator¡¨ method from 1 GHz to 13.5 GHz. The relative permittivity is calculated by observing the frequencies of resonant cavity modes. The relative permittivity of PEI/BSTZ8291 composites is almost unchanged as the measured frequency increases from 1 GHz to 13.5 GHz. The presented characteristics are better than those of polymer/BaTiO3 composites. The improvement in the tensile strength of PEI/BSTZ8291 composites may be caused by the increased interactions between neat PEI and BSTZ8291 ceramic powder, and no phase separation phenomenon occurred. The Young¡¦s modulus of the PEI/BSTZ8291 composites is improved by about 58% as the content of BSTZ8291 filler from 0 to 50 wt% and the elongation at break of the composites decreases as the content of BSTZ8291 filler increases, indicating that the composite becomes somewhat brittle as compared with neat PEI. PEI/BSTZ8291 composite substrates are developed for the applications of circularly polarized (CP) antennas. A CP antenna with a simple structure is developed as the ultra high frequency (UHF) band radio frequency identification (RFID) reader application. The fabricated antenna has an impedance bandwidth spanning from 901 to 949 MHz, which covers the entire band of Taiwan UHF-RFID frequency. The measured return loss, Smith chart, axial ratio, radiation patterns and CP gain characteristics of antennas fabricated on PEI/BSTZ8291 composites are excellent in the band of Taiwan UHF-RFID frequency. It is demonstrated that the CP antenna fabricated on PEI/BSTZ substrate has the better characteristics and small size than those fabricated on FR4 substrate.