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Nature of carbon nanotubes (CNTs) varies depending on extend of nanotubes surface oxidation. Chemical oxidation is one of the most widely applied methods in altering the undesired properties of CNTs such as chemical inertness and seamless surface nature. In this research, various oxidizing agents include HNO3, H2SO4, HNO3/H2SO4 and KMnO4 were investigated to assess their effectiveness on covalent functionalization of multiwalled CNTs (MWCNTs). The effects of different oxidizing agent on incorporation of functional groups and development of new defects on nanotubes sidewall are closely associated with their oxidation behavior. FTIR, Boehm titration method, XPS, Raman spectroscopy, and TEM were performed to examine the surface chemistry, extend of oxidation, and surface morphology of MWCNTs. The results revealed that HNO3/H2SO4 mixture is most effective oxidant in incorporating functionality groups on nanotubes surface, as indicated by highest functional groups concentration of 1.625 mmol/gCNTs. The completeness of nanotubes structure was successfully preserved under aggressive treatment, although some defective cylindrical graphene sheets and surface erosion were appeared under the visualization of TEM. [ABSTRACT FROM AUTHOR]