Capacitances of a porous carbon electrode in non-aqueous electrolytes containing tetraethylamonium tetrafluoroborate (TEABF4) and a lithium salt with various compositions have been investigated for the potential use in electric double layer capacitor. In the electrolyte prepared by dissolving TEABF4 and LiBF4 into the mixed solvent of ethylene carbonate (EC) with diethyl carbonate (DMC), an activated carbon fiber (ACF) electrode exhibits a larger capacitance than in TEABF4 single salt electrolyte on cyclic voltammograms. The symmetrical capacitor cell containing the LiBF4–TEABF4 mixed salt electrolyte also exhibits larger capacitance on a constant-current test compared with that containing the TEABF4 single salt electrolyte, while the capacitance degradation is observable for this cell at a significant extent, while the test under controlled potential of the ACF electrode to −0.2 to 1.0V vs. Ag provides somewhat stable capacitance over 30 cycles.