Multi-wall carbon nanotubes were functionalized by dielectric barrier discharge plasma in air, CO2 and He (followed by exposure to NH3) and also by refluxing in concentrated nitric acid for 2 and 4 h. The influence of these treatments on structural and electrical properties of the carbon nanotubes was investigated. Fourier transform infrared technique was used to identify functional groups on surface of the carbon nanotubes. The graphitic structure of carbon nanotubes was characterized by Raman spectroscopy and the specific surface areas were measured by Brunauer-Emmett-Teller technique. The results indicated that the nitric acid treatment significantly destroys the graphitic structure of the carbon nanotubes and shortens them while the plasma treatments have very little damaging effects on the structure. In order to study effects of the utilized treatments on electrical properties, buckypapers were prepared from the carbon nanotubes and their electrical surface resistivities were measured. The measurements showed that the acid treatment increases the electrical surface resistivity of the buckypapers while the plasma treatments do not change it considerably. Among all the studied treatment methods, the plasma treatment was found to be the most efficient approach for functionalization of carbon nanotubes. (C) 2014 Elsevier B. V. All rights reserved.