Polymers filled with inorganic nanoparticles have become interesting materials as dielectrics because of their improved mechanical and electrical properties compared with the unfilled polymers and with polymer microcomposites. These improvements are mainly due to the large surface area of nanoparticles and new polymer-nanofiller interface characteristics. In the present work, polyethylene nanocomposites with SiO2 and Al2O3 nanoparticles were prepared by melt mixing. Mechanical and electrical properties of these composites were deter-mined and morphological aspects were revealed by scanning electron microscopy, wide-angle X-ray diffraction, and atomic force microscopy. The effect of nanostructure and the importance of nanofiller dispersion were analyzed in connection with mechanical and electrical properties. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 1921-1935, 2011