The application of electroluminescent devices in smart packaging crucially depends on the photoelectric properties of the substrate material. Gas sensitivity of Epsilon-Near-Zero (ENZ) characteristic in indium tin oxide (ITO) films via annealing with different gases was reported. It is more sensitive to O2 but less to Ar and H2 for the ENZ characteristic which can be deduced from the red-shift of λENZ (Epsilon-Near-Zero wavelength) from 1298 nm (as-deposited) to 1352 nm (H2), 1805 nm (Ar) and 2048 nm (O2). We attributed the sensitivity to the existence of surface structural defects and the variety of oxygen vacancy defects. To explore the details, the surface morphology, crystallization, photoelectric and dielectric properties of ITO thin films were discussed. And the surface chemical states and vibrational levels of oxygen vacancy defect were revealed by XPS and Raman spectra. The optical bandgaps were also investigated to explain the existence of defect states.