Nitriding of nanocrystalline iron was studied under the atmosphere of pure ammonia and in the mixtures of ammonia - hydrogen - nitrogen at temperatures between 350°C and 500°C using thermogravimetry and x-ray diffraction. Three stages of nitriding were observed and have been ascribed to the following schematic reactions: (1) α-Fe → γ'-Fe4N, (2) γ'- Fe4N → ε - Fe3N and (3) ε - Fe3N → ε - Fe2N. The products of these reactions appeared in the nitrided nanocrystalline iron not sequentially but co-existed at certain reaction ranges. The dependence of a reaction rate for each nitriding stage on partial pressure of ammonia is linear. Moreover, a minimal ammonia partial pressure is required to initiate the nitriding at each stage.