Initial oxidation process of silicon in UV/ozone ambient has been monitored using a multi-wavelength, in-situ spectroscopic ellipsometry. Ozone gas was chemically formed by photochemical reaction of oxygen under ulUmviolet illuimination. The oxide growth was monitored for hydrogenated silicon surfaces as functions of oxygen gas flow rate, gas pressure and wafer temperature. Initial oxidation rates were very high at almost all the temperatures. The oxidation rate was 0.2 nm/min about ten times higher than that for thermal oxidation without UV light at low temperatures. The accelerated oxidation was probably due to an electric field effect on the oxidation of back-bond silicon by active oxygen atoms included in the ozone gas.