The defect properties of nanocrystalline TiO2 were investigated by positron annihilation lifetime spectroscopy (PALS) and X-ray diffraction (XRD) as a function of annealed temperature that ranged from 300 to 850 degrees C. Below 500 degrees C, the measured positron lifetimes of tau(1) (200-206 ps) and tau(2) (378-402 ps) revealed the existence of mono-vacancy and vacancy-clusters at grain surface and in the microvoid of intergranular region. Between 500 and 750 degrees C, the phase transition from anatase to rutile was probed by the variations of positron lifetime and XRD pattern. With the increasing temperature from 500 to 850 degrees C, the positron lifetime tau(1), tau(2) and its intensity I-2 sharply decreased from 200 ps, 378 ps, and 60% to 135 ps, 274 ps, and 33%, respectively. The results clearly indicate that the mono-vacancy or vacancy-clusters at grain surface and micro-voids between the grains were annealed out during the phase transition. (C) 2012 Elsevier B.V. All rights reserved.