Ion-plating (IP) system as plasma process is expected to apply high excess energy into thin films. Excess energy of particles on IP process was defined as difference in temperature of evaporation particles and deposition particles of substrate. In this study, Fe-Mg thin films were prepared by IP process and discussed with effect of excess energy with IP process on nanostructure of thin films. Fe-Mg thin films were prepared by Bunshah's triode type IP system. Excess energy of IP process was controlled by applied bias voltage of positive electrode. Excess energy of IP process has estimated by kinetic energy and ionization rate in evaporation particles. Kinetic energy of ion and ionization rate in evaporation particles were measured by Langmuir probe and Faraday cup respectively. Excess energy of IP process was increased with increasing of applied bias voltage of positive electrode. The value of excess energy on IP process was sufficient to exceed mixing enthalpy of Fe and Mg. In X-ray diffraction analysis, crystal structures of all samples were α-Fe bcc structures of Fe-Mg solid solution alloy. This is because lattice expansion was caused by excess energy on IP process. Nanostructure of Fe-Mg thin films was changed by Excess energy on IP process. Thus, IP process can control solubility limit and nanostructure in thin films. [DOI: 10.1380/ejssnt.2009.855]