A native oxide film is grown on by anodically oxidizing in a mixed solution consisting of aqueous tartaric acid and ethylene glycol. A homogeneous oxide film up to 8000Å thick is obtained with a linear thickness‐voltage dependence of 19.5 Å/V. The as‐grown oxide is highly transparent in the near infrared and the visible light regions. The refractive index is almost constant in the near infrared region, and it weakly depends on the wavelength in the visible light region changing from 1.7 to 1.9. In the u.v. region, the oxide becomes less transparent and is completely opaque for wavelengths shorter than 0.22 μm. The oxide is very stable and almost no detectable changes regarding oxide properties, structure, and composition occur below 350°C. However, the oxide begins to decompose above 350°C first by releasing water. The oxide decomposition most rapidly proceeds around 450°C mainly by vaporizing water and arsenic oxide. It continues up to 700°C, resulting in a pure gallium oxide. The refractive index of the decomposed oxide is fairly constant around 1.5 over the wide wavelength region from visible light to near infrared. The decomposed oxide is believed to be . By heat‐treatment above 650°C, the surface of becomes rough and a new layer is formed at the interface. This interface layer is completely arsenic depleted and has a Ga:O ratio of 2:1. The highest temperature to which the anodically oxidized can be heat‐treated is approximately 650°C. As a means for increasing this temperature limit, a double oxide structure consisting of the anodic oxide film and a film is investigated. By coating with double oxide films consisting of a 500Å thick anodic oxide film and a 2000Å thick film, can be heat‐treated at 800°C for 2 hr without degradation of the surface.