Nano-sized magnesium oxide or magnesia has been reported to be useful for suppressing the space charge accumulation when it is added in epoxy resin. However, the addition of nano-magnesia to epoxy resin decreases the glass transition temperature (Tg) signficantly, which may limit industrial applications of magnesia/epoxy nanocomposites. In this work, nanomagnesia particles with an average size of 52 nm and micro-magnesia with an average size of 3.1 pm were mixed in a commercially available bisphenol-A epoxy resin. The weight ratio of nano- and micro-fillers in each composite was changed, while the total content of the two fillers was fixed at 5 weight%. As a result, it has become clear that the addition of micro-sized magnesia fillers to magnesia/epoxy nanocomposites recovers Tg back to its original high temperature. Then, a dc electric field of 30 kV/mm was applied to the samples for 20 minutes at various temperatures from 40 to 200 °C and space charge distributions were measured by the pulsed electro-Acoustic method. The results show that space charge accumulates in both vicinities of the cathode and the anode with the same polarities as those of the nearby electrodes when the temperature is 80 °C or below. However, space charges become hetero and are observed only in the vicinity of the cathode at 140 °C and above. These homocharges and heterocharges become very small when the weight content of nano-sized fillers increases, which is industrially very important. Both complex dielectric permittivity and electrical conductivity also decrease with the increase in the content of nanofillers.