The effect of magnesium concentration on the crystal structure of Mg-Ni alloy films was investigated. Mg-Ni alloy films were prepared by means of the ion beam sputtering. Incident ions were argon ions generated by the electron cyclotron resonance (ECR) source. Various concentration ratio of Mg to Ni in film were obtained by using different target area ratio of Mg to Ni. The Mg-Ni alloy films were characterized by EDX, XRD and TEM. The films contained from 30 to 60 mol% Mg were recognized as an amorphous-like structure by XRD. Their composition are corresponding to the intermetallic compounds in Mg-Ni binary system. Cross-sectional TEM observation showed that the film deposited on silicon wafer substrates had two deferent regions of crystal structures. One is the columnar structure near the substrate and another is the equiaxed-crystalline with fine grain. It was confirmed that the composition and crystal structure of Mg-Ni alloy film could be controlled by the target area ratio of Mg to Ni.