Barium strontium titanate (BST) films prepared by RF magnetron sputtering were processed in an O-2-plasma treatment system. Experiment results indicate that the leakage current and dielectric loss of the films were effectively reduced after plasma treatment. The possible mechanism may be attributed to the passivation of oxygen vacancies, which act as electric conduction paths of leakage current. The effects of the time, power and O-2-plasma pressure of oxygen plasma treatment on the dielectric properties of BST thin films were further investigated and optimized. Compared to as-deposited films, the BST films treated in oxygen plasma for 5 min, 200 W and 3 Torr demonstrated reduced loss tangent around 0.7% and leakage current density of 7.96 x 10(-9) A/cm(2). However, it was also found that excessive plasma treatment would affect the dielectric properties of BST films negatively. (C) 2010 Elsevier B.V. All rights reserved.