Different opening and modifying methods of MWNTs are studied and compared in this paper. MWNTs are dipped in a refluxed HNO3 solution. The influence of temperature, concentration and holding time on the opening and modifying of MWNTs is studied. The compared process is that MWNTs are treated by O3. The solvent that can highly dissolve O3 and disperse MWNTs is crucial. In order to prompt the reaction, 1M CF3COOH is elected as reactant medium and catalysis. The opening of MWNTs is tested by BET and confirmed by TEM. The surface structure of MWNTs treated by different processes is analyzed by FT-IR. The optimum conditions of opening and modifying of MWNTs are discussed.