Strained epitaxial Ce0.9Gd0.1O2d (CGO) thin films are deposited on MgO(001) substrates with SrTiO3 (STO) buffer layers. The strain in CGO epitaxial thin films is induced and controlled by varying the thickness of STO buffer layers. The induced strain is found to significantly enhance the in-plane ionic conductivity in CGO epitaxial thin films. The ionic conductivity is found to increase with decrease in buffer layer thickness. The tailored ionic conductivity enhancement is explained in terms of close relationships among epitaxy, strain, and ionic conductivity.