In order to investigate the embedded capacitors with higher capacitance value in the future, two technologies of increasing the dielectric constant of filler and decreasing the thickness of capacitors can be combined to develop a best method. For that, the high dielectric constant of (Ba0.8Sr0.2)(Ti0.9Zr0.1)O3 (BSTZ) ceramic powders with three different sizes (3 microm, 200 nm, and 100 nm) are used as the filler to mix with polyetherimide (PEI) to form the PEI/BSTZ composites. The dielectric constants and loss tangents of PEI/BSTZ composites with different contents of BSTZ ceramic powders are measured using the plate method. As the contents of BSTZ powders increase from 10 wt% to 70 wt%, the dielectric constants and loss tangents of PEI/BSTZ composites increase with the increase content of BSTZ ceramic powders, independent on the particle sizes. As the BSTZ content is less than and equal to 50 wt% and same loading ratio is used, the dielectric constants of PEI/200 nm-BSTZ and PEI/100 nm-BSTZ composites are large than those of PEI/3 microm-BSTZ composites. In this study, the Lichtenecker logarithmic mixing rule is also used to fit the measured results and predict the dielectric constants of PEI/BSTZ composites.