Ion nitriding is one of the most promising thermochemical surface treatments to improve surface properties such as wear and corrosion resistance and fatigue strength. The prediction of effective case depth before nitriding treatment is very important and useful particularly from the perspective of the manufacturers in practice. In this study, ion nitriding parameter (INP), which is only a function of effective case depth and activation energy, has been suggested for the prediction of effective case depth in ion nitriding and the applicability of this approach has been verified. There is a good correlation between the progress of effective case depth and a proposed time-temperature compensated parameter. The results of proof tests have indicated that a reasonable agreement with 10% deviation was found between the experimental data and effective case depth values, which were predicted by using the parametric approach of INP for the steels AISI 4140, H13 and D3. (c) 2004 Elsevier B.V. All rights reserved.