Two techniques for improving corrosion resistance in magnesium alloys were combined, coating with high purity magnesium and treating with fluoride. The specimens used were commercial AZ31B alloy. A vapor deposition technique was applied for the coating. Temperatures of an evaporation source were varied in the range of 823 to 973 K, while a temperature of the substrate, the specimen, was kept at 523 K. Then the specimen was immersed into molten NaBF₄ kept at 693 K for various durations. The corrosion resistance of the specimen was evaluated by immersion testing using a 0.02N HCl solution and also by salt spray tests. Fluoride-treated specimens coated with high purity magnesium showed superior corrosion resistance, even in the acidic solution. The fluoride films formed on the substrate were extracted and observed with a TEM. The films were composed of small grains about 0.5 to 1 μm in diameter, and no defects, voids, cleavages, or cracks were observed. Most of the diffraction patterns were indexed as an MgF₂ phase, although the fluoride layer is composed of MgF₂ and NaMgF₃ layers.