Modified activated carbon fibers (ACFs) were used as the electrodes of an electric double-layer capacitor and showed an enhanced capacitance effect after a RF-plasma treatment. The capacitance and the surface functional groups of the ACFs were studied. For the plasma-treated ACFs having a specific surface area of 1500m2 g−1, the capacitance increased by 28% compared to the untreated sample and the highest electric capacitance value of 142Fg−1 was achieved with an oxygen feed concentration of 10vol.%. The Brunauer–Emmett–Teller (BET) surface area was 2103m2 g−1, which was 34% higher than that of the untreated sample. The pore volume was similarly increased to 483.1cm3 g−1 STP, and from the pore distribution plot, quantities of mesopores of 10nm or less and micropores also increased. However, in order to enhance the capacitance, the quinone functional group had a significant influence in addition to the BET surface area. The correlation between the capacitance and the number of quinone functional groups was confirmed because quinone is an electron acceptor.