Real-time reflection high energy electron diffraction combined with Auger electron spectroscopy (RHEED-AES) was applied to investigate the oxide growth kinetics during initial oxidation on Si (001) 2 × 1 and Si (111) 7 × 7 surfaces. The correlation between the amount of adsorbed oxygen atoms obtained from O KLL Auger electron intensity and the unoxidized surface area estimated from RHEED spot intensities due to 2 × 1 or 7 × 7 structure revealed that adsorbed oxygen was able to migrate on the Si (111) 7 × 7 surface during Langmuir-type adsorption at 580°C, leading to nucleation and lateral growth of oxide as in the oxidation mode of two-dimensional oxide island growth appearing at higher temperatures, while growth of oxide during Langmuir-type adsorption on the Si (001) 2 × 1 surface at 576°C progressed at the random sites where O2 adsorption took place.