The interface structure and electrical properties of CeO2Si (111) grown by laser ablation in ultra high vacuum was investigated by high resolution transmission electron microscopy, Auger electron spectroscopy and capacitance-voltage measurements. The deposited film was single crystalline CeO2, as indicated by RHEED and x-ray diffraction observations. However, during the deposition, a reaction between CeO2 and Si occurred at the interface. This reaction resulted in the formation of an oxygen deficient amorphous CeOx layer and a SiO2 layer. Post annealing in oxygen atmosphere caused the disappearance of the amorphous CeOx and the regrowth of crystalline CeO2. The SiO2 thickness was also increased by annealing. The modified structure of CeO2/SiO2Si showed a high break down voltage, compared with the as-deposited sample. From these results, a combination of CeO2 and SiO2 can have a great potential for SOI structure.