A electrochemical capacitor (EC) composed of horizontally aligned carbon nanotube (HACNT) electrodes treated with oxygen plasma has been successfully shown to achieve superior specific capacitance. Cyclic voltammetry (CV) measurement reveals that the HACNT thin films on the Si substrate modified by the oxygen plasma with a bias power of 60 W obtain the highest specific capacitance of 6.81 mF/cm 2 , which is 5.6 times that of non-plasma-treated ones. Such improvement can be attributed to the increase ofoxygen-containingfunctional groups, such as C-OH, C=O, and COOH as well as to structural defects which remarkably increase the extra redox reactions to enhance the specific capacitance. The electrochemical stabilities of the oxygen-plasma-treated (OPT) HACNT thin films were also verified. Finally, the OPT HACNT thin films were transferred to the flexible PET thin foil and still achieved characteristics similar to those on the hard Si substrate. Even after a bending test of 200 bends with a bending radius of 1 cm and bending angle of 90°, the variation of specific capacitance is less than 3% for such flexible ECs. The electrochemical stability of the OPT HACNT thin films after mechanical bending is equal to that of the Si substrate. This shows that such OPT HACNT ECs have promise for the future flexible applications in the energy storage field.