The optical properties of barium strontium titanate {\text{(Ba}}_{\text{0 .7}}{\text{Sr}}_{\text{0 .3}}{\text{TiO}}_{\text{3}} ; BST) thin films are described. The BST thin films were epitaxially grown upon MgO (001) substrates by pulsed laser deposition. The crystallographic properties of the BST thin films were examined by x-ray diffraction. The BST thin films were highly optically transparent in the visible region. The optical waveguide properties were characterized by a prism coupling technique. An inverse-WBK method was employed to determine the refractive-index profile along the thickness of the BST films. Optical losses were measured by a moving fiber method, and the optical losses were found to be 0.93\text{dB}/\text{cm} for the {\text{TE}}_{\text{0}} mode and 1.29\text{dB}/\text{cm} for the {\text{TM}}_{\text{0}} mode at 1550\mathrm{nm} . Electro-optic (E-O) properties were measured by a phase-modulation detection method at 632.8 nm, and the BST films exhibited a predominantly quadratic E-O effect with a quadratic E-O coefficient of 6.64\times {10}^{-18}{\text{m 2}}^{}/{\text{V 2}}^{} .