Iron is the fourth most abundant element in Earth’s crust as well as the most prevalent redox-active metal in the biosphere. Microbially mediated redox processes of Fe and N cycles by nitrate-reducing Fe(II) oxidizing (NRFO) bacteria are extremely complicated. There are three different types of NRFO bacteria according to the types of metabolism, autotrophs, heterotrophs and mixotrophs. NRFO autotrophs do not require an organic electron donor as a co-substrate but use CO₂ as a sole carbon source, and nitrate reduction is coupled with the Fe(II) oxidation using Fe(II) as a sole electron donor. NRFO heterotrophs need organic electron donors to biologically reduce nitrate, but Fe(II) oxidation occurs by just chemical processes with an intermediate of nitrate reduction (nitrite or NO); hence, most NRFO heterotrophs are denitrifying bacteria. NRFO mixotrophs can directly reduce nitrate in the presence of organic electron donors via heterotrophic processes, and simultaneously, the co-existing Fe(II) can also donate electrons to nitrate via autotrophic processes. To understand the processes of NRFO well, model strains with different types of metabolism were used to study the elementary reactions and N/O isotope fractionation of nitrate and nitrite in different conditions. Autotrophic NRFO enrichment culture KS, heterotrophic NRFO bacteria Pseudogulbenkiania sp. strain 2002 and mixotrophic NRFO bacteria Acidovorax sp. strain BoFeN1 were used, and a simple model was established to evaluate the relative contributions of the involved chemical and biological reactions. The results showed no nitrite was detected during the processes of NRFO mediated by culture KS, and nitrate reduction and Fe(II) oxidation were mainly supported by biological processes. For strain 2002, nitrate reduction was mediated by biological process and Fe(II) oxidation was mediated by chemical processes. For strain BoFeN1, the electrons used to reduce nitrate were from both of Fe(II) and acetate, indicating that both biological and chemical processes were involved in. The elementary reactions, N/O isotopic fractionation and simple models would help understanding the complicated processes of NRFO in the anoxic environments.