Fullerene-activated carbon composite electrodes were prepared and their charge/discharge characteristics were studied for use in a high power electric double-layer capacitor. The capacitance of the C-60-loaded activated carbon fiber (ACF) electrodes became greater than that of the unloaded ACF at charge/discharge current densities above 50 mA/cm(2). In order to obtain a highly dispersed C-60-loaded electrode, an ultrasonic treatment was performed. The size of the C-60 agglomerate decreased from 1-2 to 0.1 mu m or less, and the capacitance of the C-60-loaded ACF electrodes increased with an increase in the ultrasonic treatment time. A higher capacitance of 172 F/g was obtained at 50 mA/cm2 on a 1 wt% C-60-loaded electrode with ultrasonic treatment, and the C-60-loaded ACF electrode also showed a higher cycle performance. (C) 2005 Elsevier Ltd. All rights reserved.