We report the optical oscillations with monolayer periodicity observed by an oblique-incidence reflectance difference (OIRD) technique on the epitaxial growth of Nb-doped SrTiO3 on SrTiOs substrate. The periodicity was verified by the simultaneously measured reflection high-energy electron diffraction intensity oscillations. The OIRD oscillation damps during deposition, but can recover after the growth is interrupted for some time. We interpret the optical oscillations as a result of the periodic changes of the surface morphologies due to the two dimensional layer-by-layer growth of thin films.