Abstract: Carbon nanotubes (CNTs) were modified by Ar plasma followed by O₂ etching in a plasma assisted chemical vapor deposition apparatus. The modified CNTs were polymerized with aniline in a HCl aqueous solution to produce CNT/polyaniline composites. The CNTs modified by the plasma have a higher conductivity (2.86S/cm) than those modified by acid oxidation (0.936S/cm). The optimum plasma parameters were power (50W), system pressure (0.08torr), treatment time (5min), temperature (110℃), distance between two electrodes (5cm) and current of magnetic loop (0.8A). SEM results revealed that the smooth surface of the CNTs became rough after the plasma modification and the diameters increased by about 80 nm. FTIR, Raman and XRD all revealed the presence of polyaniline on CNT surfaces.