Thermal interface compositions contain both non-electrically conductive micron-sized fillers and electrically conductive nanoparticles blended with a polymer matrix. Such compositions increase the bulk thermal conductivity of the polymer composites as well as decrease thermal interfacial resistances that exist between thermal interface materials and the corresponding mating surfaces. Such compositions are electrically non-conductive. Formulations containing nanoparticles also show less phase separation of micron-sized particles than formulations without nanoparticles.