A nano-composite material is attracting many researchers' attention in the field of dielectric and electrical insulation. The purpose of this study is to investigate the influence of MgO filler size on electrical properties of MgO/LDPE nanocomposite. The volume resistivity, DC breakdown strength and the impulse breakdown strength of low-density polyethylene (LDPE) containing nano-filler or micro-filler of magnesium oxide (MgO) were measured. The volume resistivity of the sample without the MgO filler increases by addition of the MgO nano-filler or the MgO micro-filler. The impulse breakdown strength and the DC breakdown strength of the sample without the MgO filler measured with McKeown type electrode increases by the addition of the MgO nano-filler but that does not increase by the addition of the MgO micro-filler. From the result, it becomes clear that the addition of the MgO nano-filler is suitable for the DC insulating material rather than that of the MgO micro-filler.