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 1500 m 2 g(-1), the capacitance increased by 28% compared to the untreated sample and the highest electric capacitance value of 142 F g(-1) was achieved with an oxygen feed concentration of 10 vol.%. The Brunauer-Emmett-Teller (BET) surface area was 2103 m(2) g(-1), which was 34% higher than that of the untreated sample. The pore volume was similarly increased to 483.1 cm 3 g(-1) STP, and from the pore distribution plot, quantities of mesopores of 10 nm 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. (c) 2004 Elsevier Ltd. All rights reserved.