The effects of thiodiphenol based epoxy resin on the thermal property of the composites containing aluminum oxide were investigated. The thermal conductivities and thermal diffusivities of the composites were dependent on the polymer resin. At the same content of Al₂O₃, the thermal conductivity and thermal diffusivity of composites made by synthesized polymer resin exhibited higher than that of composite made by bisphenol-A resin. The obtained maximum thermal conductivity of the composite was 0.25 W/mK, which was 0.4 times as large as that of the conventional bisphenol-A epoxy resin which has an amorphous structure. It is supposed that the reason for the high heat conductivity obtained in thiodiphenol based epoxy resin is certain high packing structure.