Carbon nanotubes were modified by RF (radio frequency) oxygen-plasma at different power levels and times, and subsequently annealed at different temperatures. At the 1000th cycle of potential cycling, maximum specific capacitance of 128 F g(-1) was obtained in 0.1 M H2SO4 and with better plasma treatment conditions (power=100 W and time=30 min) and annealing temperature (350 degrees C). These show its high electrochemical stability and good specific capacitance at a higher sweep rate of 100 mV s(-1). In addition, the higher the power and time, the higher the carbonyl functional group (C=O), leading to higher specific capacitance. (C) 2008 Elsevier B.V. All rights reserved.