Single crystal epitaxial and polycrystalline thin film capacitors consisting entirely of perovskite oxides (SrRuO3/ BaxSr1-xTiO3 (20 nm)/SrRuO3) were fabricated on SrTiO3 and Si substrates by r.f. magnetron sputtering. The dielectric constants for single crystal epitaxial and polycrystalline capacitors were observed to be 681 and 274, respectively; this large difference in dielectric constants is suggested to be due to lattice distortion and grain boundaries. In spite of a thin thickness for BaxSr1-xTiO3 (20 nm), low leakage current densities for both capacitors (< 1 X 10(-7) A cm(-2) for a bias of +/-1.2 V) were observed, which is suggested to be due to high lattice matching at the interface between dielectric and electrode. In the microstructure of the polycrystalline capacitor, BaxSr1-xTiO3 (BSTO) and SrRuO3 (SRO) formed continuous columnar grains and showed epitaxial growth at the BSTO/SRO interface within each column; this microstructure was modeled as a 'local epitaxial film' which was situated between single crystal epitaxial and polycrystalline films.