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 128Fg−1 was obtained in 0.1M H2SO4 and with better plasma treatment conditions (power=100W and time=30min) and annealing temperature (350°C). These show its high electrochemical stability and good specific capacitance at a higher sweep rate of 100mVs−1. In addition, the higher the power and time, the higher the carbonyl functional group (CO), leading to higher specific capacitance.