The application of the multiple-angle incident ellipsometry to measure the ultrathin (<100 A) oxide has been studied in this paper. First, four interfacial models are investigated by using a fitting scheme to fit ellipsometric data (A, ψ) at various incident angles, and the abrupt model is found to be the most appropriate model to model the transition region of the Si02-Si interface. The sensitivities on the incident angle and the errors induced by the ellipsometric parameter variations are also analyzed. Ellipsornetry is applied to measure the native oxides of Si wafers after they are treated with different cleaning processes, and it is also applied to measure refractive indexes and thicknesses of ultrathin thermally grown Si02. It is believed that these are the most accurately measured results on the refractive indexes of ultrathin oxides. Also, an empirical formula for thermal oxide growth in dry 02 is obtained.