The strain in Si, on which a single-crystalline CeO2 gate oxide was epitaxially grown, was investigated by evaluating lattice spacings in CeO2 and Si. In-plane X-ray diffraction measurements and observations of electron diffraction patterns by a transmission electron microscope were performed to examine the lattice spacings precisely. It was found that the lattice spacings in epitaxial CeO2 isotropically expanded by; 1%, compared with those in bulk polycrystalline CeO2. The oxygen-defect-induced state was observed in the CeO2 valence bandedge by X-ray photoelectron spectroscopy measurements. The decrease of the coulombic interaction in ionic oxide due to the oxygen defects may induce the expansion of the lattice spacings in CeO2. It was clarified that Si at 50 nm depth from the CeO2/Si interface was tensile strained owing to the expansion of the lattice spacings in CeO2. Oxygen defects in epitaxial crystalline gate dielectrics must be controlled by taking account of Si channel properties. (C) 2004 The Electrochemical Society.