Composite samples of low-density polyethylene (LDPE)/nano-SiO2 and LDPE/micro-SiO2 were prepared with the method of double-solution mixture. Depolarization currents of all samples were investigated with thermally stimulated depolarization current (TSDC). It was found that the currents of both composites increased with the loading level of nano-SiO2 and/or micro-SiO2, and that the peak width of each composite is greater than that of pure LDPE. In addition, the peak position of the nanocomposite shifts as the loading level increases, while that of the microcomposite does not shift significantly. In order to understand activation energy of both composites and pure LDPE, the initial-rise method was used to analyze the depolarization current. It was found that LDPE has the greatest activation energy among all samples and the activation energy of both composites decreases with increasing loading levels. Moreover, the activation energy of the nanocomposite is less than that of the microcomposite at each of the same loading level. As the nano-SiO2 loading level reaches 5.0%wt, the composite has the lowest activation energy of 0.25 eV. In addition, dielectric spectra of all samples were investigated in the range of 10−4 to 107 Hz, and it was found that the peak position of loss tangent varied consistently with the TSDC curves as the loading levels of nano-SiO2 and/or micro-SiO2 were increased. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.