Activated carbon aerogels (ACAs) with hierarchically porous structures and high specific surface area have been prepared via CO2 and KOH activation processes. The pore structures of ACAs are characterized by N2 adsorption/desorption and scanning electron microscopy. The experimental results show that the ACAs contain three types of pores: micropores with diameters below 2nm, small mesopores with diameters from 2 to 4nm and large pores or channels with diameters over 30nm. The typical sample ACAs-4, which possess pore volume of 2.73cm3 g−1 and specific surface area of 2119m2 g−1, exhibits high specific capacitances of 250Fg−1 and 198Fg−1 at the current densities of 0.5Ag−1 and 20Ag−1 respectively in 6M KOH aqueous solution. Furthermore, the resultant ACAs electrode materials also exhibit high power density, good cycling stability and long lifetime. With these features, ACAs are expected to be promising electrode materials for electrical double layer capacitors.