Er2O3-Al2O3 film was deposited on the Si(001) substrate by radio frequency magnetron technique at room temperature.The samplewas annealed at 450,600 and 750 oC for 30 min in O2 ambience,respectively.The optical constants were studied by spectroscopic ellipsometryfor both the as-deposited and the annealed samples.The proper values of refractive index indicated that it could be a useful material for solar cells. Er2O3-Al2O3 film was deposited on the Si (001) substrate by radio frequency magnetron technique at room temperature. The sample was annealed at 450,600 and 750 oC for 30 min in O2 ambience, respectively. The optical constants were studied by spectroscopic ellipsometry for both the as -deposited and the annealed samples. The proper values ​​of refractive index indicated that it could be a useful material for solar cells.