Nitriding is usually applied to increase the surface properties of mechanical components and can also enhance the fatigue resistance. The aim of this paper is to investigate, by means of numerical models and experimental tests, the effects of residual stresses induced by nitriding on the fatigue behavior of a marine diesel engine crankshaft. The residual stress gradient induced by the thermochemical treatment was taken into account by means of finite element models. Experimental tests were carried out with an axial testing machine in order to validate the numerical models and assess the crankshaft mechanical parameters such as the yield strength and the fatigue limit. An experimental innovative method applied to evaluate the crankshaft residual stresses by means of strain measurements under bending was also developed. This methodology proved to be useful to determine the magnitude of the residual stresses induced by the thermal treatment into the crankshaft, and it could be applied for the evaluation of the residual stress field in several cases.