Recently, research works for electrical properties on the elastic epoxy have been also progressing to diagnose insulation materials by development of measure technology. The epoxy which specially applied for high voltage instruments have required for the electrical, mechanical and thermal properties. Nanocomposites were made from insulating material epoxy resin using for power transformer as varying g amount of addition of SiO2, we found the change of physical characteristics of nanocomposites by using SEM and XRD. To investigate the electrical properties according to the amount of addition and temperature, tested insulting breakdown in silicon oil. As the result of insulting breakdown testing, the insulting breakdown strength increased according to change of the amount of addition.