Ion plating (IP) process is a kind of vacuum evaporation process. In this study, Bunshah's triode type ion plating system was used for the film preparation. In this process dense plasma flux (～2.5 A) of ionized source materials can be dosed and deposited on a substrate. The flux of the source vapor is ionized by thermal electrons accelerated from melted evaporant to a positive electrode so called``probe''. Excess energy of vapor particles is able to control by applied bias voltage of``probe''. Fe has no Mg solid solution phase in equilibrium phase diagrams. In this study, Fe Mg thin films were prepared by an IP process. Investigated the relation between crystal structure and excess energy of particles with bias voltage. In all samples, a Fe body centered cubic structure was observed by X ray diffraction patterns. The lattice distortion changed with a change in applied bias voltage. These results were suggested that the excess energy of vapor particles was inputted more than the mixing enthalpy of Fe and Mg. At the same time lattice distortion energy in Fe increased with increasing Mg content. Film structures of samples were dependent on the kinetic energy of evaporated vapor particles changed by bias voltage.