The dielectric function transformation of ITO thin films caused by different substrates and post annealing temperatures is studied by spectroscopic ellipsometry measurement using Drude and Tauc-Lorentz combined modes. By comparing with the Hall effect measurement results and optical bandgap values calculated from transmittance and reflectance spectra, it is found that the influences on the dielectric functions from the carrier concentration and optical bandgap values occur at infrared and ultraviolet wavelength regions, respectively. The relationship between the dielectric functions and carrier concentration and optical bandgap is deducted by studying the dielectric functions at low and high energy regions. This study provides a new technological way to analysis the electrical and optical properties of ITO thin films by non-contact spectroscopic ellipsometry measurements.