Spectroscopic ellipsometry (SE) was used to characterized sol-gel Ba0.7Sr0.3TiO3 (BST) thin films on Pt- coated Si. Four samples were investigated, each containing a sol-gel BST thin film with a nominal thickness of 200 nm deposited on Pt and annealed at 400, 500, 600 and 700 degree(s)C, respectively. In this study, our main objective was to determine the temperature-dependent optical properties of the BST thin films. In the analysis of the measured spectra, two fitting models were used, one containing a BST film and another containing a surface roughness layer and a BST film. The optical properties of BST film in both models were represented by a Cauchy's function. Our SE analysis clearly shows that the refractive index increases with the increase of the annealing temperature, which, we believe, is an indication of further crystallization of BST thin film. In addition, we also determined the BST film thickness for each sample and found it decreasing with the increase of the annealing temperature.