Thickness of compound layers formed on the surface of pure iron during the nitriding process was analytically calculated and compared with experimental data in the gaseous and plasma nitriding. Plasma nitriding was carried out on a high purity iron substrate at a temperature of 550oC in an atmosphere of 75 vol. % H2 and 25 vol. % N2 for various nitriding times. The thickness of compound layers was evaluated by several characterization techniques including optical microscopy, SEM and XRD. Using the Fick's first diffusion law and a mass conservation rule, two separate equations were developed for predicting the thickness of the binary compound layers; epsilon (ε) and gamma prime (γ'), in terms of the nitriding process parameters. The results of modelling indicated good agreement with the experimental data, provided that appropriate correction factors are applied. The flexibility and reliability of the models were increased by introducing two factors, Kε and Kγ'; the calculated curves corresponded well with both gaseous and plasma nitriding experimental data.