Atomic structures on active oxidized silicon films by an ozone gas at room temperature are investigated by an atomic force microscopy. A step-terrace structure similar to that on a clean Si (001) prepared by a silicon homoepitaxy is preserved on the ozone oxidized surface. These atomically regulated structures are also discernible on the SiO2/Si interface when a 1.0-nm-thick SiO2 film oxidized by an atmospheric ozone is removed by a diluted HF etching. It is revealed that the homogeneous lateral oxide growth, i,e., layer-by-layer growth, proceeds at room temperature by an active oxidant such as ozone.