Structural and optical characterization of mechanically milled ZnO powders are presented in this paper. It is shown that the application of mechanical milling is a very effective and simple technique to produce nanocrystalline powders, with the possibility of obtaining large quantities of materials. The milled powders are analyzed by X-ray diffraction, positron annihilation spectroscopy, scanning electron microscopy (SEM) and photoluminescence spectroscopy (PL). As milling proceeds, a clear reduction of grain size and homogenization are observed.