Thin films of vanadium oxide were prepared and studied for the electro-optical properties of semiconductor-metal transition. Vanadium oxide films with thickness of ~ 0.15μm were deposited on SiO2/Si substrates by reactive RF magnetron sputtering using a pure vanadium target under various ratios of argon and oxygen gases. The oxygen content in the mixed atmosphere has a significant influence on the semiconductor-metal transition characteristics. Both thermochromic and electrochromic modes were studied. In thermochromic mode, the oxide film deposited in an O2/Ar ratio of 1.2% exhibits 90% optical transmission in semiconducting state at room temperatures, while very low transmission at 5% in metallic state at 65°C, in the wavelength region of 8 to 12μm. In the near infrared region of 1 to 2μm, the transmission is about 60% in the semiconducting state and a few percents in the metal state. A corresponding three-order variation of resistivity was observed over the transition. The refractive indices (n and k) of the vanadium oxide films were measured using an ellipsometer in the near infrared region between 1 and 2 μm in both states. The index n decreases in metal state while k increases. The electrochromic phase transition of vanadium oxide was investigated by applying a pulsed voltage to minimize the heating effect. The required charge density for the phase transition is consistence with the Mott metal-insulator model. Longwave IR switching and modulation were demonstrated by electrically induced semiconductor-metal transition.