Via properly changing concentration of titanium precursor, tuning the residence time and controlling of the temperature field in the reaction region, nanocrystalline TiO2 powders were synthesized by the diffusion flame method with well-dispersed behavior and controlled crystal structures and particles size. The XPS and IR results showed that there existed high hydration and oxygen vacancies on the surface of the as-prepared nanocrystalline TiO2 powders, which played a dominant role in controlling polymorph fraction by altering the critical particles size of phase transition. The thermodynamics analysis demonstrated that the high hydration on TiO2 surface can produce the lattice contraction, which effectively stabilizes anatase phase. However, the oxygen vacancies on TiO2 surface can decrease activation energy of phase transition of anatase to rutile, which facilitates the phase transformation. (c) 2007 Elsevier B.V. All rights reserved.