The influence of annealing temperature on the micro-structure, surface morphology and optical property of TiO2 thin films prepared by mid-frequency ac magnetron sputtering technique has been studied. The micro-structure and surface morphology were examined by X-ray diffractometer (XRD) and atomic force microscopy (AFM). The transmittance and reflectance spectra of TiO2 thin films on fused silica substrate were measured by spectrophotometer. The reflective indices and extinction coefficients of TiO2 films were calculated by envelope method and experimental expression, respectively. The results show that the TiO2 thin film sputtered at room temperature is amorphous. After annealing under lower temperature (=700 degreesC), anatase phase appeared in TiO2 thin film. After annealing under higher temperature (=900 degreesC), the structure of TiO2 thin film changed rutile completely; the TiO2 grains changed from column to nubbly. The refractive index of TiO2 thin film increases with annealing temperature and the extinction coefficient decreases a little during the lower temperature anneal. The TiO2 thin film annealed at 500 degreesC has the best optical property. (C) 2003 Elsevier B.V All rights reserved.