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The microwave dielectric properties of binary composites made by high dielectric constant ceramic fillers and polyethylene matrix are studied in this paper. Three ceramic powders of fillers with dielectric constants of 100 and 170, respectively, are adopted. The experimental dielectric constants of ceramic dispersions in the polyethylene matrix at microwave frequencies of 5 to 10 GHz are compared to those obtained by using different mixing laws. Six powder mixture rules derived from three basic filler particle shapes with random distributions; as well as six exponential and the logarithmic mixture laws are investigated. The mixing rules are also adopted to estimate the dielectric constants of pure ceramics from the measured dielectric constants of composites with various concentrations. The theoretical error of each law for estimation is studied. Two new exponential rules are proposed. It was found the rule derived from filler particles of prolate spheroid with low filler concentration and exponential rules with the order of 1/3 to 1/5 have better estimation accuracy.