In this study, the capacity change of supercapacitor was investigated by surface treatments of carbon nanotubes as electrode materials with various methods, such as ball-milling, $KMnO_4$ and $H_2SO_4/HNO_3$ acid mixture. Surface treatments generated a number of defects on the surface of carbon nanotubes by attacking on $pi$ bond in graphene layer, at which carboxyl groups were introduced. These hydrophilic groups could enhance the capacity by increasing the wettability of carbon nanotube surfaces. However, a drawback of the surface treatment was the decrease of conductivity by the loss of conduction path in graphene layer due to the defect formation. The surface treatment condition should be therefore optimized between hydrophilicity increase and conductivity decrease.