The surface chemistry of ITO thin-films before and after UV-ozone treatment was characterized using angle dependent Xray photoelectron spectroscopy (ADXPS). After solvent cleaning, the ITO surface was covered with a thin nonconducting carbon contamination layer of similar to7 Angstrom. This contamination layer was removed efficiently by UV-ozone treatment, and the chemical states of the residual carbon at ITO surface after UV-ozone treatment were quite different from contaminated carbon. UV-ozone treatment modified ITO surface by introducing O2- ions into ITO surface. The modified depth was about 50 Angstrom. The modification decreased the carrier concentration at ITO surface, and thus decreased the conductivity of ITO surface.