Electrochemical reduction of indium tin oxide (ITO) in SiO2 solution was investigated by analyzing the properties of ITO and the reactant diffusion in ITO films. In the electrochemical system, an ITO anode and cathode were immersed in 1 wt % SiO2 solution (pH 6.8). The electrodes were set to 15 V at room temperature under different durations of electrochemical treatment. The properties of the cathodic ITO film were analyzed after treatment. The ITO films varied from transparent to black and gray during the treatment. In2O3 underwent reduction and transformed to In, as indicated by X-ray diffractometry. The surface morphologies of the ITO samples revealed that the ITO crystals were damaged. The damage resulted in numerous aggregated particles forming on the film surface after electrochemical treatment. The major elements of the particles were confirmed to be oxygen and In. The electrical resistance of the treated ITO considerably increased because of the damage to the In2O3 crystals. The simulation of the diffusion model and experimental results indicated that the electrochemical reaction rate of ITO was controlled by the reactant diffusion in the ITO film.