The influence of gas and plasma nitriding on thermal fatigue resistance of a 41CrAlMo7 steel is considered. The role of compound and diffusion layers is discussed on the basis of the damage observed in service. The thick and porous compound layer, resulting from gas nitriding, is not very effective in preventing thermal crack nucleation and propagation up to the interface with the underlying diffusion layer. On the contrary, the thin and compact compound layer obtained by plasma nitriding is able to prevent crack nucleation. Overnitriding, i.e. excessive nitriding induced embrittlement, has to be avoided in order to obtain high crack arrest fracture toughness in the diffusion layer. This suggests the need to realize surface layers with optimized hardness. A simplified approach to predict the conditions for crack propagation in the diffusion layer as a function of the thermal loading and the properties of the nitride layer has been also proposed.