The current paper presents a new approach of attaining the optimum grinding condition of a planetary ball mill and simple method for purifying multi-walled carbon nanotubes (MWCNTs) to investigate the dispersion characteristics of MWCNTs. This work was conducted under dry and wet grinding conditions at various rotation speeds (200 rpm-500 rpm), and the dispersion characteristics of MWCNTs in aqueous solutions with and without surfactant were studied. The results were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle-size analysis (PSA), UV-spectrophotometry and zeta potential (zeta) measurements. The purification results show that the structures of amorphous carbon and carbon particles of MWCNTs were completely eliminated and the tips of nanotubes opened. Moreover, both dry and wet grinding caused the lengths of the MWCNTs to be shortened with increasing rotation speed. The maximum absorbance of nanofiuid was revealed to be 2.485 abs at wavelength of 253 nm for the best dispersion. The best dispersion characteristics were observed for wet grinding at a rotation speed of 500 rpm assisted by the ultrasonication dispersion of CNTs in aqueous solutions with surfactant. (C) 2012 Elsevier B.V. All rights reserved.