Activated carbon modified by Radio Frequency (RF) plasma at different power levels, times and volume flow rates of oxygen was prepared for activated carbon electrodes. Electrochemical characterization of different activated carbon electrodes was carried out using cyclic voltammetry (CV) with different electrolytes at a concentration of 0.1 M. A maximum capacitance of 38.9 F g−1 was obtained in a 0.1 M H2SO4 solution for the activated carbon (precursor material: coconut shell made in Japan) with plasma treatment conditions: power = 300 W, time = 3 min, and volume flow rates of oxygen = 45 sccm (standard cubic centimeter per minute). In addition, specific surface areas/pore-size distributions, functional groups, and surface morphologies of activated carbon with/without plasma treatment were examined by gas adsorption meter, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscope (SEM), respectively.