Abstract: X-ray Diffraction (XRD), small-angle X-ray scattering, scanning electron microscopy and energy dispersive X-ray Analysis were used to investigate the effect of controlled high energy ball milling (HEBM) on the average volume weighted crystallite size, < D >(V) and weighted average microstrain, < epsilon >, of nanostructures of CuO prepared by solid state reaction. The starting material, S-0, consists of almost strain free nanocrystals of monoclinic CuO with < D >(V) & 20nm, as determined by XRD data Rietveld analysis. It was found that after an initial decrease of < D >(V) and increase of < epsilon >, the values of these parameters go through a steady-state stage followed by an increase of an order of magnitude in < D > after a period of only 120 m of HEBM. According to the results here presented, the presence of small amounts of contaminants in the starting material can have an influence on the kinetics of crystal growth in HEBM CuO Ver menos