Pure zinc oxide (ZnO) nanocrystalline was successfully prepared by the mechanochemical method followed by a heat-treatment using ZnCl2 and Na2CO3 as reagents, while NaCl was used as a diluent. The samples were characterized by Transmission Electron Microscopy (TEM) and Xray Diffraction analysis (XRD). The effects of the milling time and the heat-treatment temperature on ZnO nanocrystalline size were investigated in this work. Experimental results show that increasing the milling time can effectively reduce the nanocrystalline size of ZnO. The average crystal size is about 21 nm for the ZnO nanocrystallites by the calcination of the after-milled powder at 600 degrees C in air and removal of NaCl through washing. The crystal size increases with increasing heat-treatment temperature. The mechanism of nanocrystallite growth is discussed. (c) 2006 Elsevier B.V. All rights reserved.