Fe/N ultrafine powder is prepared by the method of one-step gas phase synthesis, using FeCl2 and NH3 as raw materials, and gamma -Fe4N ultrafine powder is prepared by twice-nitriding the powder above. The processing and technique of preparing nitrided iron ultrafine powder by the twice-nitriding method was studied. Results show that gamma -Fe4N can be produced. The physical properties of Fe/N and gamma -Fe4N powder, such as crystalline state, physical phase, shape, chemical composition and magnetic properties are characterized by means of XRD, TEM, XPS and VSM. The experimental results show that gamma -Fe4N powder prepared by twice-nitriding is highly pure, ultrafine and single phase. The value of saturation magnetic susceptibility sigma (s) and coercive force H-c of this powder is preferable to Tanaka's corresponding study results [T. Tanaka, Japan Patent, 60-111403, 1985.].