Perovskite CaTiO3 has three polymorphic phases: cubic, tetragonal and orthorhombic. The present article deals with the first time preparation of nanocrystalline orthorhombic polymorphs of CaTiO3 by high energy ball milling the equimolar mixture of CaO and anatase (a)-TiO2 powders. It has been observed that a small amount of CaTiO3 is formed just after 1 h of ball milling and the wt.% of the phase increases with increasing milling time. The starting mixture is transformed almost completely to nanocrystalline CaTiO3 within 6 h of ball milling. To verify the stability of the phase at high temperature, the 6 h milled powder has been post-annealed at 773 and 1273 K for 2 h. Phase transition kinetic study, quantitative phase estimation and microstructure characterization of CaTiO3 and other secondary phases evolved at different duration of ball milling in ball milled samples have been critically analyzed by Rietveld's powder structure refinement method using X-ray powder diffraction data. Nanocrystalline (similar to6 nm) CaTiO3 powder has been obtained within 1 h of milling but in the course of milling up to 6 h of duration, crystallite size is seen to increase gradually with increasing milling time due to agglomeration of nanosized crystallites. It has been observed that the a-TiO2 has been transformed to polymorphic srilankite(s)-TiO2 phase and nanosized CaTiO3 crystallites are formed by the solid state diffusion of nanosized s-TiO2 and CaO crystallites. Post-annealing study reveals that the orthorhombic CaTiO3 phase is stable up to 1273 K with larger and strain free crystallites of similar to125 nm. (C) 2004 Elsevier B.V. All rights reserved.