Nanosized TiO2 thin films were prepared by the method of DC magnetron sputtering,and the films were deposited on alumina tubes and silicon substrates. After the deposition, the films were annealed at 500℃, 700℃ and 1 100℃ in a muffle respectively. The film properties such as crystal structure, grain size, phase and gas sensitivity varied on account of the annealing condition. X-ray diffraction (XRD) and gas sensing measurements were carried out to find out the relation between sensitivity and annealing temperature. Anatase occurs at annealing temperature 500℃. Sensitivity is the highest with wonderful selectivity and shortest response(recover) time for the anatase phase. After modified the surface of TiO2 thin film,it could be concluded that the best operating temperature for the devices is 370℃. The mechanism of the gas sensing properties was also discussed.