The （001） and （111）-oriented MgO buffer layers, on which ferroelectric Ba0.7Sr0.3TiO3（BST30） thin films were prepared subsequently, have been grown on Si （001） single-crystal substrates by sol-gel and magnetron sputtering methods. The microstructure of the thin films was investigated by XRD, AFM and SEM. The results showed that the （101）-prefer-oriented BST30 thin film was obtained on the thicker MgO（001） buffer layer, while only the competition between the （101） and （111） orientations of BST30 can be observed on the thinner MgO（111） buffer layer. With the increasing thickness of MgO（111） buffer layer, the BST30 thin film’s （101） orientation was restrained, whereas the （001） orientation was strengthened. The improved simplex method was utilized to obtain the optical constant （n） and thickness （d） of BST30 thin film and MgO buffer layer. It was also found that the MgO buffer layer can significantly decrease the leakage current and p-n junction effect of BST30 thin film.