Ba_0.9Sr_0.1TiO₃(BST) thin films were deposited on LaNiO 3 thin films by modified chemical solution routes. X-ray diffraction investigation shows that BST thin films are highly preferential (1?0?0)-orientation. Atomic force microscopy shows that the films have large size of 80～200 nm. The optical properties of the films were measured by spectroscopic ellipsometry in the photon energy range of 0.7～3.4 eV. Cauchy model was used to express the optical properties of BST thin films. Optical constants (refractive index n and extinction coefficient k) of the BST films were obtained. The band gap E*g of the BST films was determined to be 3.36 eV.