Al2O3-AlN/silicone rubber composites with high thermal conductivity were prepared by using Al2O3 and AlN as thermally conductive fillers. By using theory of granulartity, we obtained the densest packing of fillers in the polymer composites. The results showed that the thermal conductivity of the composites reached up to 9.6 W/(m?K) at the loading of 96 wt%, which was 60% higher than that filled with Al2O3. The significant increase of the thermal conductivity of the composites resulted from the increase of the internal thermal conductivity of the sample. Although the composites have undergone extreme cold and hot cycle application conditions, the thermal conductivity of the composites was maintained well, showing excellent reliability.