Indium Tin Oxide (ITO) films are highly transparent in the visible region, exhibiting high reflectance in the infrared region, and presenting nearly metallic conductivity. Owing to this unusual combination of electrical and optical properties, this material is widely applied in optoelectronic devices. In this study, ITO films were deposited on Corning Glass substrates by DC magnetron sputtering technique at 100 watts using an ITO ceramic target (In2O3:SnO2, 90:10 wt%) in argon atmosphere at room temperature, and a posterior annealing at different temperatures in oxygen-rich atmosphere. The electrical, optical and microstructural properties of ITO films, as a function of the post-annealing temperature were analyzed by XRD, spectral, Hall Effect and AFM measurements. It has been found that as-deposited the amorphous ITO films transform to a polycrystalline structure at temperatures above 200oC. At the same time, the resistivity decreases and the visible optical transmittance increases. The best electro-optical parameters of the ITO films have been obtained by annealing at 300oC in pure oxygen environment. This temperature allows for the fabrication of high quality ITO films on flexible polymer substrates Kapton from DuPont. The use of other flexible substrates, such as Teflon, which are important for thin film photoelectric applications, is connected with the desire of decreasing the annealing temperature below 200oC; a possible solution to achieve this goal is discussed as a future work.