We have discussed a cross-linked polyethylene (XLPE) nanocomposite insulating material that is able to DC voltage applications. Nanocomposites, which are composed in polymer matrix mixed with nano-fillers, have received considerable attention because of their potential benefits as dielectrics. The nano-sized alumina oxide (Al 2 O 3 )/XLPE nanocomposite was prepared, and three kinds of test, such as DC breakdown, DC polarity reversal breakdown, and volume resistivity were performed. By the addition of nano-sized Al 2 O 3 filler, both the DC breakdown strength and the volume resistivity of XLPE were increased. A little homogeneous space charge was observed in Al 2 O 3 /XLPE nanocomposite material in the vicinity of electrode through the polarity reversal breakdown test. From these results, it is thought that the addition of Al 2 O 3 nano-filler is effective for the improvement of DC electrical insulating properties of XLPE.