This study presents the results of a comparative analysis of the properties of nitrided layers obtained by gas and ion nitriding of structural alloy steels used to manufacture critical, highly loaded parts of marine diesel engines. The paper presents a comparative study of wear resistance, fatigue strength and cavitation resistance of alloy steels after gas and ion nitriding; the advantage of the latter method in terms of these characteristics is shown. The distribution of residual stresses after gas and ion nitriding is studied. A technological process for ion nitriding of marine diesel engine parts made of 38CrAl, 38Cr2MoUАl and 18Cr2Ni4MoАl steels has been developed, which has advantages over classical technologies of chemical-thermal treatment of the surface of the parts being strengthened.