Epitaxial conductive RuO2thin films were used as bottom electrodes of Ba0.5Sr0.5TiO3(BSTO) thin-film capacitors. The BSTO showed pure (111) orientation normal to the substrate surface when grown on (200) oriented epi-RuO2on LaAlO3. The BSTO film consisted of a main body of stoichiometric bulk layer and a slightly oxygen deficiency region on the very film surface as revealed by Rutherford backscattering spectrometry. The dielectric constant of BSTO was above 400 at 10 kHz with zero dc electric field bias. A bias dc electric field modulation of the dielectric constant was observed to be more than 80% at 100 kHz at a field intensity of 6 × 105V/cm.