The optical properties of Bi3.25La0.75Ti3O 12 (BLT) and Bi 3.25Nd0.75Ti3O12 (BNT) thin films de posited on (111)Pt/Ti/ SiO2/Si substrates by a chemical solution method were investigated. B oth BLT a nd BNT thin films showed single phase of bismuth-layered structure. The BNT fil m composed of the homogeneous and large rod-like grains, while the grains in BLT film were small. The optical constants (refractive index and extinction coeffic ient) in the wavelength range of 200—1700nm and the thickness of the films we re obtained by spectroscopic ellipsometry measurements. The optical band gaps o f BLT and BNT were estimated to be 430 and 361eV, respectively. The disper sion of the refractive index in the interband transition region was analyzed by using the single electronic oscillator model.