•Severe shot peening prior to nitriding, caused deeper compound and compressed layer.•Severe shot peening after nitriding, caused remarkable increment of surface residual stress.•Notwithstanding the high surface roughness, severe shot peening improved the fatigue limit by 11.6%.•Nitriding improved the fatigue limit by 51.3%.•Combination did improve surface characteristics. It did not succeed to improve fatigue limit. Fatigue strength of mechanical components can be greatly enhanced by generating compressive residual stress, increasing the hardness and reducing the grain size. It is well known that while the use of mechanical treatments is able to generate an effective field of compressive residual stresses and, if severe parameters are used, to cause grain refinement, thermochemical treatments are able to increase the surface hardness. This justifies the interest in developing combined treatments, able to achieve all the just mentioned factors. In this study, the effect of combination of severe shot peening and nitriding on the fatigue limit of a low-alloy steel is investigated. Severe shot peening was conducted by using particular processing parameters to obtain ultra-fine/nano-structured surface layers. Micro-structural observation, micro-hardness, surface roughness and XRD measurement of residual stress were performed on single and hybrid surface treated specimens including nitrided, severely shot peened, nitrided plus severely shot peened and severely shot peened plus nitrided specimens. The fatigue limit of all series was experimentally determined and compared with the as-received specimens. Severe shot peening and Nitriding improved the fatigue limit by 11.6% and 51.3% respectively. Combination of severe shot peening and nitriding improved hardening, residual stress and nitrogen diffusion with respect to the single treatment. Nevertheless, it could not guarantee further improvement in the fatigue limit as compared with the nitrided smooth specimens. The results are critically assessed by considering the local fatigue limit concept.