In order to form atomically flat silicon oxide films, it is essentially important to use atomically flat silicon substrates and the layr-by-layr oxidation process. First, 200-nm-thick thermal oxide films were formed in dry oxygen at 1000ﾟC.Second, this oxide films were removed by buffered hydrofluoric acid. Third, on silicon substrates thus obtained silicon films were epitaxially grown by the gaseous reaction of SiHCL_3 at 1100ﾟC followed by the cooling in hydrogen gas until the substrate temperature decreases to be smaller than 400ﾟC.the silicon substrates thus obtained were maintained in nitrogen gas. Fourth, hydrogen-teminated Si(III)-1*1 and Si(100)-2*1 were obtained by removing native oxides, which were grown in nitrogen gas during relativelely long storage time, in dilute hydrofluoric acid. Hydrogen-terminated Si(III)-1*1 surface was also prepared by treating silicon substrates, which were obtained after the second procedure described above, in 40% NH_4F solution. In order to form uniform oxide film, the 0.6-nm-thick preoxides were grown in 1 Torr dry oxygen at 300ﾟC without breaking Si-H bonds. Through this preoxide thermal oxides were grown in 1 Torr dry oxygen at more than 600ﾟC.The height of protrusions on thermal oxide films thus formed on Si(III) and Si(100) surfaces are smaller than the height of two atomic steps, that is, 0.314 nm and that of single atomic step, that is 0,135 nm, respectively. Especially, the extremely flat oxide films were formed on Si(100). From the correlation between surface microroughness and SiO_2/Si interface structures the SiO_2/Si(100) interface is expected to be extremely flat and can be the ultimate interface which will be used in the most advanced MOSFET in the future. These results were obtained from the atomic-scale observation of surface morphologies of oxide films using noncontact-mode atomic force microscope and the measurement of interface structures using X-ray photoelectron spectroscopy.