Microstructures of the plasma nitrided ferritic Fe-M (M = 0.5, 0.7, 1.0mass%Ti, 1.0mass%V, 1.0mass%Al) binary alloys were studied mainly by TEM. Metastable clusters are uniformly formed in the Ti added alloys. Near the surface of the specimen, transition from the clusters to the equilibrium B1-type TiN is observed. The microstructure formed in the 1%V alloy is similar to that of the Ti added alloy. In contrast, no cluster is formed in the 1%Al alloy and B1-type AIN is observed to nucleate on dislocations.