The physicochemical properties of carbon materials are strongly influenced by the presence of chemical species on the surface. In this study, surface functional groups on cup-stacked-type carbon nanotubes (CSCNTs) were introduced by gas phase, liquid phase, and electrochemical oxidations. Several techniques were used to characterize these materials including nitrogen adsorption, chemical and thermal analyses, FT-IR, XPS, SEM, TEM, and Raman. The results indicated that the amount of surface functional groups on CSCNTs was strongly affected by the oxidation method. In particular, the electrochemical oxidation of CSCNTs was effective and increased the concentration of carboxylic acid.