AISI 4340 steel was ion nitrided using different time and temperature and the gathered properties were assessed by evaluating hardness profile, compound layer thickness, case depth and surface roughness by using microhardness tester, optical microscopy, surface profilometer and X-ray diffraction. It has been found that the thickness of the compound layer increases with increasing treatment time and temperature. The surface roughness, the surface hardness, the hardness profile, the case depth, the compound layer thickness, and phase content of the compound layer depend on nitriding parameters such as treatment time and temperature. Optimum ion nitriding parameters of component should be determined depending on the service conditions. For this reason, the effect of ion nitriding treatment parameters on structural properties of AISI 4340 steel was investigated. (C) 2011 Elsevier Ltd. All rights reserved.