Highly concentrated aqueous electrolytes are expected to be applied to electric double layer capacitors (EDLC) because of the wide potential windows, environmental superiority, and low cost. In this paper, the dependence of discharge characteristic on current density was investigated using saturated LiClO4 and NaClO4 aqueous solutions as electrolytes. As a result, the saturated NaClO4 solution showed better discharge performance compared with the saturated LiClO4 solution. This is well correlated with the fact that the hydration radius of Na+ is smaller than that of Li+ in terms of stokes radii. The discharge capacitance was measured in aqueous solutions under various concentrations of NaClO4 and was found to decrease about 5% with the increasing concentration of NaClO4 from 10 to 17 m (m = mol/kg). This result is considered to be due to the formation of uncharged complex of [Na(H2O)n-1ClO4], where ClO4－ coordinated at the first coordination sphere of Na+, under high concentrations of NaClO4. 23Na and 17O-NMR measurements gave strong supports for the formation of uncharged complex, which could act negative role for the capacitance.