The struclUral evolution of nanocrystaUine Ti02 was studied by X-ray diffraction (XRD) and the Rietveld refinement method (RRM). Ti02 powders were prepared by the sol-gel technique. Post annealing of as·synthesized powders in the temperature range from 500°C to 600 °C under air and argon atmospheres led to the formation of Ti02 nanoparticles with mean crystallite size in the range of 37-165 nm. based on the Rietveld refinement results. It was found that the phase structure, composition, and crystallite size of the resulting particles were dependent on not only the annealing temperature, but also the annealing atmosphere. Rietveld refinement of the XRD data showed that annealing the powders under argon atmosphere promoted the polymorphic phase transformation from anatase to rutile. Field emission scanning electron microscopy (FESEM) was employed to investigate the morphology and size of the annealed powders.