The nitric acid oxidation followed by sodium hydroxide washing was applied for functionalization of singlewalled carbon nanotubes (SWCNTs), few-walled CNTs (FWCNTs), and multiwalled CNTs (MWCNTs). The surface functionalization of CNTs via acidic oxidation was studied by transmission electron microscopy, Raman scattering spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. The oxidation of CNTs involves the exfoliation of functionalized carbon layers into smaller carbonaceous fragments (CF), which might influence the performance of composite materials based on modified CNTs. Optimizing oxidation time increased the selectivity to thermodynamically unstable functionalized CNTs. Thicker CNTs, including FWCNTs and MWCNTs, are better for high-density surface functionalization, due to higher structure stability. It was proposed that a rational selection of the oxidation process and type of CNTs is important for the applications involving CNT-related composite materials.