An ion beam sputtering (IBS) system has been developed to synthesize various magnesium alloy thin films. Different from the conventional coating methods suffering from the high-energy ion bombardment or high heat flux damage, this technique is free from those radiation damages in coating. In addition, any substrates, rather weak to radiation and heating damages, can be used; this process can be just called "Cold Coating" in the present paper. This technique has a significant advantage in practice, being free from an ion collision, a thermal effect or the radiation damage from the generated plasma. Incident argon ions to the target were generated by an electron cyclotron resonance (ECR) source. In order to control the concentration ratio of transition elements (TM) to magnesium in film, the target area ratio was changed with the. combination of pure metal target plates. Deposited magnesium alloy films were characterized by EDX, XRD and TEM. Various concentrations of Mg alloy films were obtained with 10 to 90 mol% TM. Through XRD analysis, an amorphous-like thin film can be fabricated. Cross-sectional TEM observation showed that the Mg-Ni film deposited on silicon wafer substrates had two different regions of crystal structures. One is a columnar structure near the substrate and the other is an equiaxed-crystalline with fine grain. It was confirmed that the composition and crystal structure of magnesium alloy films could be controlled by the target area ratios of TM to Mg.