A single-phased Ag2O film was deposited on glass substrate by direct-current reactive magnetron sputtering, and was then vacuum thermally annealed at different annealing temperatures (T A) for 1 hour. Effect of the TAon the films microstructure and optical properties was investigated by X-ray diffractometry, scanning electron microscopy and spectrophotometry. The results indicate that Ag nano-scaled particles begin to appear in the annealed Ag2O film at TA= 300 ℃. The Ag content obviously increases with increasing TA, and in particular, Ag2O phase is completely transformed into Ag at T A = 475 ℃. The evolution of the films surface morphology from dense to loose indicates that the diffusion and escape of O atoms from film surface accompanied the thermal decomposition reaction of Ag2O to Ag particles during the vacuum thermal annealing. The changes of the films transmissivity, reflectivity and absorptivity with T A are attributed to the thermal decomposition of Ag2O and the films structure evolution during annealing.