We have investigated the depth profile of the oxygen content of indium tin oxide (ITO) films fabricated by reactive sputtering. The oxygen content was determined by means of O-16(alpha,alpha)O-16 resonant backscattering. The effect of the oxygen partial pressure of reactive gas on the oxygen content of the films was investigated. From the depth profile of oxygen, it was found that the oxygen compositions of the films, especially in the surface region, changed with the oxygen partial pressure. Also it was found that the electrical and optical properties of the ITO films changed with the oxygen partial pressure. The correlation between the oxygen content and the electrical and optical properties is discussed.