ITO/ Si gas sensor was fabricated by an efficient and size – controlled by using DC-Sputtering technique suitable for large deposition area and high quality thin films. Structural, optical and electrical properties of ITO thin films were investigated and analyzed extensively under different doping concentration. Structure and surface morphology of ITO thin films were characterized by X-ray diffraction, Atomic Force Microscope. XRD technique that showed these films is polycrystalline structure with a preferred orientation of (222),( 440),(400) the best orientation plain is (222) It was found that crystalline size decreases with increasing doping concentration. The optical properties of ITO thin films were studied such as transmissions, energy gap The transmittance was measured in the wavelength range from(300nm to 900 nm) for all the films it was highly transparent (greater than 85%). The optical energy band gap was increased with doping concentration in range from (3.9 to 4.15) eV. The electrical properties for ITO thin films include D.C electrical conductivity and Hall effect which shows that the type of films is (n- type), and the film has two activation energies in the rang (305-355)K, and the resistivity increases with doping concentration at 8wt%. The sensitivity toward NH3, NO2 gas has been measured.In2O3 doped with (Sn) has higher sensitivity to NH3, The sensitivity toward, NO2 gas has been measured, where In2O3 doped with (Sn) has higher sensitivity to NO2than to NH3.