Ion nitriding is a glow discharge surface modification technique, which is primarily used to increase the fatigue strength, wear, corrosion resistance and surface hardness of steels. Because of the formation of high compressive residual stresses in the case region, increasing surface hardness and case depth cause remarkable improvement in fatigue properties of steels. In this study, the ion nitrided properties of quenched and tempered AISI 4340 low alloy steel were investigated under different process parameters including time and temperature. It has been found that the ion nitriding surface treatment improves the fatigue strength and increases the fatigue limit depending on the case depth. Up to 91% improvement in fatigue strength of the steel has been attained by ion nitriding. It was determined that, the subsurface 'fish eye' type formation is the dominant fatigue crack initiation mechanism in ion nitrided AISI 4340 steel in high cycle region and its origin was nonmetallic inclusions. (c) 2007 Elsevier Inc. All rights reserved.