This article describes an application of reflectance-difference (RD) spectroscopy, that is a surface-sensitive optical technique, to monitoring of the oxidation process on Si. The RD spectrum of a single-domain Si(001) surface shows repeated inversion of its polarity as oxidation proceeds in the layer-by-layer mode. Taking advantage of this oscillation, we have demonstrated in situ measurements of the oxide thickness with atomic-layer precision, which allows the determination of oxidation rate for each monolayer of Si. Such layer-resolved kinetic data will expectedly advance our understanding of the atomistic mechanisms of the oxidation processes.