The capacitance and electrochemical stability of three kinds of activated carbon fibers (ACFs) with different micropore widths (0.87, 1.09, 1.30 nm) were evaluated as the electrodes of electric double layer capacitors using three sulfone-based electrolytes; sulfolane (SL), isopropyl methyl sulfone, and ethyl methyl sulfone solutions containing tetraethylammonium tetrafluroborate. The effect of ion sieving on the capacitance of the ACF electrodes and the rate performance in the SL electrolyte were more sensitive to the micropore structure compared to the conventional propylene carbonate (PC)-based electrolyte. Charge-discharge cycling tests using a two-electrode cell with the ACF electrode with the widest micopore width and a sulfone-based electrolyte were carried out with a charge cut-off voltage up to 4 V to evaluate the electrochemical stability of the electrolytes. The SL electrolyte showed a lower degradation during cycling than the PC electrolyte and the other sulfone-based electrolytes even when charged to a 4 V cut-off.