The surface modification of multi-walled carbon nanotubes (MWCNTs) by O-2 plasma was carried out in this study. In order to achieve a relatively homogeneous treatment of MWCNTs powder, a rotating barrel fixed between the two discharge electrodes was used. The effect of plasma treatment parameters, such as power, time, and positions of samples (inside and outside the barrel), on the morphology and structure of MWCNTs surface was systematically analyzed by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The results showed that the direct discharge (outside the barrel) could result in not only a quick grafting of polar functional groups but also an easy damage of MWCNTs after longer time, particularly under intensive power. It was found that the surface of MWCNTs powder might be changed in three steps-expansion (loosed structure formed), peel off and oxidization with increasing of treatment time during the irradiation. In this way, a complete purification of MWCNTs powder could be finished within 30 min via plasma treatment. Our work suggested that plasma treatment could be a simple and nonpolluting method for a large scale purification of MWCNTs. (c) 2007 Elsevier B.V. All rights reserved.