CNTs was opened and surface modified by K2Cr2O7 and HNO3. The ratio of CNTs weight loss was used to show the effect of oxidation and the morphologies of CNTs was observed by TEM and IR. The results showed that the concentration of K2Cr2O7, oxidation temperature and oxidation time all affected the oxidation results. Among these three factors, oxidation temperature produced the largest effect; As the CNTs was oxidized, the wall of CNTs was corroded firstly and made the wall of CNTs thinner, then the curved-site of CNTs was attacked by oxidant and made the CNTs break in several section. So the solution would enter into the cavity of CNTs. Hydrophilic groups would be introduced in the inner and outer surface of CNTs with the oxidation reaction continuing. It would be good to the compatibility of CNTs with other materials.