Ammonia is the only inorganic nitrogen molecule which can be metabolized by organisms. Most of higher plants and microorganisms which are not provided with the ability of N2 fixation, therefore, reduce NO3- and NO2- to NH3 (assimilatory reduction). On the other hand, some bacteria reduce those ions to N2 (dissimilatory reduction). Thus, the amounts of inorganic nitrogen molecules are regulated by nitrogen cycle, where molybdenum and iron atoms play key roles as the active center. This review describes chemical simulation of nitrogen cycle catalyzed by not only Mo complexes but also MoFeS and FeS clusters.