Oxidation of silicon in a radio frequency discharge oxygen plasma was studied usinginsituduring process rapid ellipsometry (1 s resolution and interval). Plasma characteristics were also determined by the Langmuir probe method and emission spectroscopy. From the measurement ofinsituduring process rapid ellipsometry, oxidized film thickness of silicon increased steeply right after starting rf discharge and slowly after ≊3000 s. It was found that thickness change rate measured byinsituduring process rapid ellipsometry immediately after starting the rf discharge was strongly proportional to O2+ions density.