Plasma nitriding behavior of iron alloys was studied from the view point of material science. The role of nitriding of iron or steels in the field of surface modification was clarified and then the features of plasma nitriding were discussed compared with those of the conventional gas nitriding. The strengthening mechanism of nitriding layer and the kinetics of its growth were discussed on the basis of results of the plasma nitriding of ferritic iron alloys, Fe-Ti and Fe-19Cr alloys. The strengthening of an internal nitriding layer due to dispersion of fine nitride precipitates was interpreted by either the cutting model or by-pass one in dislocation-precipitate interactions. The mechanism depends upon the volume fraction of the pre-cipitates. The growth rate of the nitriding layer was discussed on the basis of nitrogen diffusion in the iron matrix on the analogy of internal oxidation. The plasma nitriding of the austenitic iron alloy brought to make clear the role of plasma. The collision of N₂+ ions to the specimen surface prevented the external layer of γ'-Fe₄N from forming because of sputtering. And then the exited species in the plasma succeed to react directly with the specimens. This direct reaction caused a remarkable excess amount of nitrogen to dissolve into austenitic iron phase. Finally, an application of plasma nitriding to powder materials was discussed.