The surface reaction kinetics of Si thermal oxidation was investigated by a real-time monitoring method of Auger electron spectroscopy combined with reflection high-energy electron diffraction. From the time evolution of O KLL Auger electron intensity measured simultaneously with that of RHEED intensity, thermal oxidation on the Si(001) surface under an O₂ pressure of 2×10^-7 Torr was divided into three temperature regions: (1) Langmuir-type adsorption at T < 630^oC, (2) two-dimensional (2D) SiO₂ island growth at 630^oC < T < 800^oC, and (3) etching (active oxidation) at 800^oC < T. In the temperature region of 2D SiO₂ island growth, an oscillatory behavior of RHEED half-order spot intensity of (1/2, 0) and (0, 1/2) was observed, indicating layer-by-layer etching of the surface between SiO₂ islands. The RHEED intensity oscillation was accompanied with an appearance of bulk diffraction spots in RHEED patterns, suggesting a development of protrusions under the SiO₂ islands, however no bulk diffraction spots were observed in other two regions. On the basis of the correlation between SiO₂ coverage and RHEED intensity of specular, half-order and bulk diffraction spots, the time evolution of surface morphology is discussed for Langmuir-type adsorption and 2D SiO₂ island growth.