An energy storage system consisting of electric double-layer capacitors was designed for hybrid electric vehicles (HEV). The input energy and the CO2 emission for the capacitor system were compared with those for a lithium ion battery system. A vehicle of 2, 000 cc class was assumed to run in the 10 . 15 mode defined by Japanese Industrial Standard (JIS). From the result of calculations, the CO2 emission in the life-cycle model for HEV decreased by 35.7% with increasing system capacity from 666 Wh to 903 Wh, even though the CO2 emission increased with system capacity at the time of energy storage system manufacture. The superiority of the capacitor energy storage system was clarified by evaluation over the whole of the life cycle model.