Ferroelectric thin films are currently being used to develop tunable microwave circuits based on the electric-field dependence of the dielectric constant. Perovskite Ba0.5Sr0.5TiO3 thin films were deposited on Pt/Ti/SiO2/Si and LSCO/Pt/Ti/SiO2/Si substrates by radio frequency magnetron sputtering. Extensive X-ray diffraction studies exhibit good in-plane relationship of [100]Ba1-xSrxTiO3 (BSTO)//[100//[La0.5Sr0.5CoO3 (LSCO). At the applied voltage of 7 V, the tunability values of barium strontium titanate (BST) films grown on Pt and LSCO/Pt were nearly 2.2:1 and 2.8:1, respectively. The change of dielectric properties in BST/LSCO and BST/Pt is attributed to the change in the film stress and the film orientation. (C) 2002 The Electrochemical Society.