Nanocrystalline vanadium pentoxide has been processed with high energy planetary ball-mill at two different ball-to-powder mass ratio (BPMR) for different lengths of time (0-35 h). Microstructure characterization of ball-milled samples is made by employing Rietveld's X-ray powder structure refinement analysis in terms of lattice imperfections. The particle sizes and lattice strain values are found to be anisotropic. The plastic deformation induced in the process of ball-milling caused a massive decrease in particle size and increase in lattice strain within 15 min of milling, and a saturation of particle size of similar to6.5 nm is reached within 10 h of milling. The shape of milled particle is presumed to be parallelepiped and the influence of BPMR predominated in initial size reduction. The initial change in intensities of two major X-ray reflections is found to be due to insignificant change in atomic coordinates on the event of severe high energy impact. The peak broadening at initial stage of milling is due to fracture of particles, but later (after particle size saturation), dominated by increase in anisotropic lattice strains particularly along [0 0 1]. (C) 2002 Elsevier Science B.V All rights reserved.