Compared with the inorganic particles/silicone rubber (IOP/SR) composites filled with single-sized particle solely, the IOP/SR composites cofilled with micro-sized Si3N4 and nano-sized Al2O3 particles show a low relative dielectric permittivity (5.25) and high thermal conductivity (1.62W m 1K 1), which are a bit higher than the permittivity (4.1) and remarkably larger than the thermal conductivity (0.16W m 1K 1) of pure SR. Agari model gives a better prediction for these results. The images indicate that the nano-sized Al2O3 particles are closely filled into the gaps between the micro-sized Si3N4 particles and play a role as a bridge-link action to form a continuous thermally conductive network. This also confirms that the loading of the micro-sized Si3N4 particles in the SR composites prepared by the cofilled model contributes a positive thermal conduction and induces the low dielectric permittivity of the IOP/SR composites