Carburizing treatments are of prime importance to developing fatigue and wear characteristics of steels. It is also well known that the relative case depth, which considers the size of components as well as the depth of carburized case is one of the most important parameters determining fatigue performance. In this study, the effect of relative case depth on fatigue performance of AISI 8620 steel has been investigated by conducting a series of rotary bending fatigue tests on 10 mm diameter specimens having carburized case depths between 0.73 and 1.10 mm. An attempt was made to construct empirical relationships between the parameters defining relative case depth and fatigue strength of the steel based on failure at 10(6) cycles, and it was concluded that a power model could be used to represent the relationship between case profile and fatigue performance of carburized specimens within the testing limits of the study. (C) 1999 Elsevier Science Ltd. All rights reserved.