The effect of annealing in atmospheric environment on Ta2O5 thin films was researched, which were prepared by ion beam sputtering. The annealing temperatures ranged from 150 ℃ to 550 ℃, and the interval was 200 ℃. The optical band gap (1-4 eV) of the Ta2O5 thin film was characterized by the Cody-Lorentz dielectric model. And the microstructure vibration was characterized by the oscillator model in the range of infrared region(400-4 000 cm-1). The results show that the turning point of the annealing temperature appeared between 150 ℃ and 350 ℃. The extinction coefficient of the thin film increased when the annealing temperature was above the turning point. The variation of Urbach energy was in accordance with the extinction coefficient, but the variation of band gap was opposite. By analyzing the microstructure vibration in infrared, it′s found that the stoichiometry defect of the protoxide was in the Ta2O5 films.