The properties of silicone rubber filled with three kinds of binary mixtures of alumina particles with different size distribution (i.e., 30 mu m + 0.5 pm, 10 mu m + 0.5 mu m, and 5 mu m + 0.5 mu m) were investigated as a function of relative volume fraction of the 0.5 mu m particles in the hybrid alumina (V-s) at a fixed total filler content of 55 vol%. The results indicate that each binary mixture of alumina-filled silicone rubber exhibited improved thermal conductivity and tensile strength, and decreased dielectric constant, compared to a single particle size filler-reinforced one, and the maximum improvements were obtained at the V-s ranging from 0.2 to 0.35; the coefficient of thermal expansion (CTE) of filled silicone rubber obviously reduced with increase in the V-s, whereas the elongation at break slightly decreased. At V-s = 0, the larger particles-filled silicone rubber showed higher thermal conductivity, CTE, dielectric constant, and elongation at break, and lower tensile strength compared with the those of the smaller particles-filled one.