Sample bias dependence of plasma oxidation rate of silicon has been studied using real-time ellipsometry and probe measurements. The results reveal that the rate determining process in the ultra thin oxide regime (≤2nm) is different from that in the thick oxide regime (>2nm). In the ultra thin oxide regime the oxidation rate is enhanced not only by positive bias but also by negative bias. The enhancement of the oxidation rate is explained mainly in terms of the collision of electrons with the adsorbed oxygen species by positive bias and also of the collision of cations to surface by negative bias. In the thicker oxide regime (>2nm) the oxidation rate is enhanced only by positive bias and is constant at the negative bias. This reflects that anions and/or neutral species contribute to the oxidation of silicon in the thicker oxide process. The plasma oxidation rate as a function of sample bias exhibits a minimum at around +27V, which corresponds to the plasma potential for short oxidation time, and starts to rise at about +10V, which corresponds to the floating potential for long oxidation times.