To understand the properties of the composite insulating materials, DC breakdown test and the thermal conductivity measurement of polymethylmethacrylate (PMMA) / Aluminum oxide (Alumina) composite material produced by the electrostatic adsorption method were performed. In the PMMA/Alumina sample which has an Alumina as a main particle and a PMMA as an adsorption particle, DC breakdown strength decreased with the increase of Alumina content. It was considered that the increase of the interface between an alumina particle and a PMMA particle led to the lower DC breakdown strength. The thermal conductivity in PMMA/Alumina composite material increased with the increase of Alumina content. From these results, it was suggested that the electrostatic adsorption method could produce the various structure in the material and the insulating materials with various properties like the higher thermal conductivity and the tolerable breakdown strength by using the combination like the particle size of a main particle and adsorption particles.