Ion nitriding has become a popular thermo-chemical surface treatment, which is being used to develop fatigue and wear characteristics of steels. Besides the increased strength, the formation of high compressive residual stresses in the case region causes remarkable improvement in fatigue properties of steels. In this study, the effect of case depth on fatigue performance of AISI 4140 low alloy steel has been investigated by conducting a series of rotary bending fatigue tests at the frequency of 95 Hz, on hour glass shaped, 4 mm diameter specimens, which had been ion nitrided at 748 K for 1, 3, 8 and 16 h. Up to 50% improvement in fatigue strength of the steel has been attained by ion nitriding, depending on the case depth formed during the process. The comparison of test results between ion and liquid nitrided specimens having the same case and core properties, has shown that a 12% increase in fatigue strength could be reached by ion nitriding with respect to conventional, salt bath nitriding process. An attempt was made to establish some empirical relationships between the parameters defining relative case depth, which consider case depth and specimen size together, and fatigue strength of ion nitrided steel. II was obtained that the subsurface 'fish eye' type crack formation is the dominant fatigue crack initiation mechanism in ion nitrided AISI 4140 steel, and a map has been constructed to illustrate the locations of crack origins depending upon the case depth and the cyclic stress acting on specimens. It was also tried to explain the conditions promoting the 'fish eye' crack formation by analysing the combined effects of residual and applied stress patterns as well as inclusion size. (C) 2000 Elsevier Science S.A. All rights reserved.