In this paper, the (Ba0.8Sr0.2)(Ti0.9Zr0.1)O-3 (BSTZ) ceramic powder is used as filler to format the polyetherimide (PEI)/BSTZ composites. The relative permittivity of PEI/BSTZ composites is measured at the range of gigahertz (GHz) using a "square cavity resonator" method. Two different sizes of cavity resonator are used to measure the relative permittivity of PEI/BSTZ composites. The measured relative permittivity of PEI/BSTZ composites is almost unchanged as the frequency increases from 1 to 13.5 GHz and the sizes of the cavity resonator are changed. It is concluded that PEI/BSTZ composites can be used as a microwave substrate with good dielectric characteristics. (C) 2008 The Electrochemical Society.