Abstract: The modification of multi-walled carbon nanotubes(MWCNTs) was carried out with a mixed acid of concentrated sulfuric acid and nitric acid at a volume ratio of 3:1.The morphology,specific surface area and surface functional groups of the resulting MWCNTs were analyzed by field emission scanning electron microscopy(FE-SEM),surface area analysis and X-ray photoelectron spectroscopy(XPS),respectively.The electrochemical properties of the MWCNTs were studied by cyclic voltammetry and galvanostatic charge-discharge tests.The results indicate that after oxidation by acid,the lengths of the MWCNTs are shortened,and the specific surface area is enlarged and the contents of surface nitrogen-and oxygen-containing functional groups increase.This leads to the enhancement of electrochemical properties of the MWCNTs,and the specific capacitance of MWCNTs can increase from 7 F·g^-1 to 66 F·g^-1.