We fabricated an epitaxial Au/SrTiO3(001)/SrRuO3 (or CaRuO3)(001)/SrTiO3 (or LaAlO3)(001) heterostructure to investigate the effect of the strain on the epitaxial tunable dielectric thin films. SrTiO3 thin films showed very different dielectric properties depending on the bottom electrode with an opposite lattice mismatch. The SrTiO3 thin films grown on the CaRuO3 bottom electrodes showed nearly a two times larger tunability than that on SrRuO3. We think this is due to the different strain on the SrTiO3 thin films. The tensile strain along the applied electric field in the parallel plate capacitor enhances the dielectric constant and the tunability, while the compressive strain decreases them. We believe this is consistent with the hardening of the soft mode phonon due to the compressive strain. © 2001 American Institute of Physics.