The effect of addition of nano-sized magnesia fillers to epoxy resin on the distribution of space charges is examined at various temperatures from 40 to 200 °C. Although the spatial distribution of space charges in the sample varies depending on temperature and filler content, magnesia nanocomposites exhibit a smaller amount of homocharge near the cathode than the unfilled epoxy resin at 40 and 80 °C. At 120 and 140 °C, although the polarity of space charge in the vicinity of the cathode becomes positive or hetero, its amount becomes smaller with the increase in content of magnesia nanofillers. This finding agrees well with the fact that magnesia nanofillers effectively suppress the formation of space charge in low-density or crosslinked polyethylene. As another distinctive feature, it has become clear that the addition of magnesia nanofillers effectively suppresses carrier transport, especially at high temperatures.