The atomic-scale surface roughness of thermal oxides formed on Si(100) were studied as a function of oxide film thickness up to 3.15 nm using non-contact mode atomic force microscopy (NC-AFM). The height deviation of the oxide surface is limited to within a single atomic-step height of 0.135 nm on a Si(100) surface below the thickness of about 1 nm, but above this thickness, it increases with an increase in thickness at 700 degrees C and then decreases drastically by the oxidation at 900 degrees C and becomes comparable with the surface roughness before the oxidation. Oscillations in surface roughness with constant amplitude and period intervals of 0.19 nm were observed in the thickness range studied. Surface morphologies are affected strongly by the oxidation temperature, while the interface structures are affected weakly by the oxidation temperature. The area where the layer-by-layer oxidation reaction occurs is on the order of 5 nm in diameter at 700 degrees C and increases significantly with an increase in oxidation temperatures up to 800 degrees C and 900 degrees C.