Two kinds of low-density polyethylene (LDPE) composites were prepared by adding semi-spherical MgO fillers different in diameter, one being around several ten nra and the other being several um, and their dielectric properties were compared. At all the measurement temperatures, the conductivity is decreased and the permittivity is increased by the addition of fillers, irrespective of their sizes. However, the conductivity decrease is more significant and the permittivity increase is suppressed more in the case of the nm fillers. In addition, a drastic increase in dielectric loss factor at low frequencies is suppressed more significantly by the addition of the nm fillers. While the formation of packet-like charge observed in LDPE, which is assumed to appear at the front of the high-conductivity region that proceeds from one electrode toward the counter one, is suppressed significantly by both fillers, the effect is more significant in the nanocomposites. This means that the nanofillers suppress the increase in conductivity induced by the voltage application more effectively.