The silica-overlayer deposited on MgO (SiO_x/MgO) is prepared by the chemical vapor deposition of tetramethoxysilane. The surface property of the resulting oxides is characterized by XPS, IR, TPD and titration techniques. The deposition of silica overlayer brings about a change in acid-base property of the surface or the concentration of surface hydroxyls. The generation of Lewis acid sites on SiO_x /MgO is evidenced by 1R spectra of adsorbed pyridine. The amount and the strength of acid of SiO_x /MgO are found to be greater than those of silica gel. With increasing silanization period (the amount of deposited SiO_x), the amount and the strength of acid increase at coverages less than monolayer, while the multilayer deposition of SiO_x layer decreases the surface acidity. XPS study shows that the chemical state of Si atoms in SiO_x overlayers is different from that in silica gel. It is concluded that the generation of acid sites on SiO_x/MgO results possibly from the electronic and structural effects from the MgO substrate through Mg-O-Si bond formed at the interface.