The influence of ion nitriding with surface roughness and initial microstructure on fatigue behavior and mechanical strength of AISI 4340 steel have been investigated under different process parameters including time and temperature. The first group fatigue and tension test specimens were not heat treated before ion nitriding and received in hot-rolled condition while the second group specimens were quenched and tempered. While the surface of the reduced section was being mechanically polished for the fatigue specimens, some specimens in the second group were not mechanically polished before ion nitriding to obtain the third fatigue test specimens group. While the effects of initial microstructure, surface roughness and ion nitriding conditions on fatigue behavior were investigated, the effects of initial microstructure and ion nitriding conditions on mechanical properties were investigated. First improvement in fatigue and mechanical properties were obtained with quenched and tempered. The obtained case depth of QT AISI 4340 steel is thicker than that of the hot-rolled AISI 4340 steel at all ion nitriding conditions and a linear relationship was obtained between fatigue strength of steel and case depth for both steels. Although the subsurface fatigue crack formation is dominant in failure initiation mechanism in ion nitrided steels, surface roughness effect on fatigue strength continued in a decreasing rate. (c) 2012 Elsevier B.V. All rights reserved.