The evaluation of the growth kinetics of Fe4N1-x (gamma') phase under microwave post-discharge nitriding in ARMCO iron has been carried out through this project. A mathematical model that considers the iron nitride growth is proposed in a non-saturated nitrogen medium with a finite length of 2L, where the interface concentrations remain constant during the treatment. Growth kinetics of gamma', during microwave post-discharge nitriding process, is rapid in short times of treatment, due to the presence of neutral and ionized species, which produce a rapid evolution of nitrogen concentration on the substrate surface. Concentration on the surface is controlled by the parameters of the process for gamma' nitride formation. Experimental results achieved during the process are compared to the mathematical model forecast, along with the results presented by other authors. (c) 2004 Elsevier B.V. All rights reserved.