The main purpose of this study was to reveal usability of various characterization techniques for certain aspects of surface functionalized multi-walled carbon nanotubes. Surfaces were first oxidative functionalized by sulphuric acid/nitric acid mixture, then aminosilanized by gamma-aminopropyltriethoxysilane. Chemical groups formed on carbon nanotubes due to these surface treatments were characterized by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and also energy dispersive spectroscopy. Morphological changes and crystal structure of surface-treated carbon nanotubes were analyzed by scanning electron microscopy and X-ray diffraction, respectively. Thermogravimetric analysis was also used to observe thermal degradation of the chemical groups formed on the nanotube surfaces. In the second part of the study, Polyamide-6 nanocomposites were produced by using unmodified and surface functionalized carbon nanotubes. Transmission electron microscopy indicated that surface functionalization improves distribution of carbon nanotubes in the matrix, while flexural tests revealed that strength and modulus values could be increased as much as 30% and 40%, respectively, due to enhanced interfacial bonding between the matrix and nanotubes.