Electrophoretic deposition (EPD) of a suspension with added polydimethylsiloxane (PDMS)-based organic-inorganic hybrid materials can provide a heat-resistant, thermally conductive, and electrically insulating coating on an aluminum substrate. The addition of high-molecular-weight PDMS imparts flexibility to the resultant material, which is advantageous. However, the inclusion of PDMS with higher-molecular weight in the suspension can also affect the properties of the EPD suspension and the EPD kinetics. In this paper, the influence of the molecular weights of the starting materials of a PDMS-based hybrid (which is used as a binder) on the resultant material is investigated. These effects are examined in terms of the structure, thermal properties, thermal conductivity, and electrical insulating properties of the EPD layer, which is composed of Al2O3 and the PDMS-based hybrid materials. It is found that the weight ratio of the Al2O3 to the binder component is increased, but the addition of higher-molecular-weight PDMS reduces the density of the EPD film. Thus, Al2O3 coatings on metal substrates with good electrical insulation properties, heat conductivity, and thermal stability are fabricated via EPD, using an added PDMS-based organic-inorganic hybrid sol of appropriate molecular weight as a binding additive.