Barium strontium titanate ((Ba, Sr)TiO3) thin films were prepared on Pt/Ti/Si substrates by the sol-gel method using metal alkoxides. The dependence of the dielectric constant for the films on the film compositions and on the film thickness were investigated. The dielectric constant of the fi Im of thickness 180 nm had the highest value of 230 at composition ratios of [Ba + Sr]/[Ti] = 1.04 and [Sr]/[Ba + Sr] = 0.6. The dielectric constant of the films with this composition decreased from 390 to 160 with a decrease in the film thickness from 440 to 100 nm. The decrease in the dielectric constant with increasing film thickness is attributed to the existence of a low-dielectric-constant interface layer adjacent to the electrodes.