The thermal oxidation kinetics of Si(110) surface up to oxide layer thickness of 1 ML has been investigated by real-time monitoring of chemical shifted in the Si 2p core-level photoemission using synchrotron radiation. The uptake profiles of every Si oxidation states (Si
n+: n = 1 − 4) indicate that the top surface Si(110) oxidation proceeds through a two-step oxidation pathway via Si2+ state, just like the Si(001) surface. In contrast to the Si(001) oxidation, however, Si3+ state is always more abundant than Si4+ state during oxidation. This is related to occurrence of imperfect oxidation of this surface, most probably due to accumulation of compressive strain during oxidation.