Indium tin oxide (ITO) substrates are subjected to oxygen plasma and UV ozone treatments. It is observed that the treatments produced a surface layer rich in oxygen, as investigated by X-ray photoelectron spectroscopy (XPS), and this is correlated with the sheet resistance of ITO measured by a four-point probe. A method has been devised to measure the change in the sheet resistance more prominently and this measurement is carried out under purified nitrogen atmosphere after the ITO is being treated. With an oxygen-rich ITO surface layer formed on ITO after the oxidative treatments, the resistance of ITO is considered to be that of a parallel-resistor combination and the thickness of the surface layer is being estimated based on this approach.