Abstract: Bismuth oxide thin films were prepared on ITO glass substrate by radio frequency（RF） magnetron sputtering.X-ray diffraction、atomic force microscopy and X-ray photoelectron spectroscopy were employed to characterize the microstructure、surface morphology、composition and valence state of the thin films.It is indicated that the crystal structure of the thin film annealed at 350℃in air is tetragonal phase and the thin film shows a clear electrochromic change between transparent and dark brown on switching voltage between -1.8V and +3.0V.There are no other valence states（except the trivalent state） for bismuth during the process of coloration and decoloration,which is confirmed by the results of XPS analysis.The measurement of UV-Vis spectrophotometer demonstrates that the average optical modulation in the wavelength range from 400 to 800nm can reach 44%.The electrochromic efficiency at 550nm is 9.6cm~2/C proved by cyclic voltammetry.The results show that the Bi₂O₃ thin films possess well electrochromic property.