We have developed a high-precision difference-x-ray-reflectivity technique using intense synchrotron radiation and applied this to evaluate native oxides and ultrathin thermal oxides on Si(100). We have successfully evaluated the density of native oxides. Native oxides formed by HCl and NH4OH solutions have a low density, in contrast to the oxides formed by H2SO4 solution and UV/O-3 whose densities are close to those of thermal oxides. By carefully analyzing ultrathin thermally grown oxides with thicknesses of 40 and 70 Angstrom grown at 800-1000 degrees C, we have revealed the existence of a dense (similar to 2.4 g/cm(3)), thin (similar to 10 Angstrom) layer at the SiO2/Si interface. Oxides grown in O-3 Or HCl/O-2 have a thinner interfacial layer compared to those grown in O-2. We have evaluated the effects of ambient and temperature at oxidation on the interfacial layer and the SiO2 layer.