To improve the electrochemical performance of an activated carbon (AC)-based electric double-layer capacitor (EDLC), the AC surface, which is used as an electrode, was modified using physicochemical base tuning that uses different concentrations of ammonium hydroxide. The effect of the treatment on the surface and electrochemical properties of the AC electrodes was investigated. The specific capacitance of a 13 vol%-solution-treated sample was increased to 385 F/g at a scan rate of 5 mV/s, which was 17% higher than the value of 328 F/g obtained for the untreated samples. This increase can be attributed to an increase in the mesopore volume ratio due to the etching effect of the reaction between the carbon surfaces and ammonium hydroxide. Moreover, nitrogen functional groups, which were introduced by the treatment, also improved the electrochemical properties of the resulting AC-based electrode. Therefore, a simultaneous etching and nitrogen-introducing method with ammonium hydroxide can easily introduce nitrogen functional groups on the surface of an AC electrode. This method is very effective for preparing AC for use in an EDLC with improved electrochemical properties.