In a previous work it has been demonstrated that shot peening improves the contact fatigue resistance of a Cu-Mo diffusion bonded sintered steel by 30%. Such an improvement is due to the combination of compressive residual stresses, surface densification and strain hardening. By means of a theoretical model to predict contact fatigue crack nucleation, it has been concluded that the effect of phenomena related to plastic deformation (surface densification and strain hardening) prevails on that of residual stresses. In the present work, the study was extended to other materials, having a lower resistance to plastic deformation resulting from either a lower density or the presence of diffusion bonded Ni. Even in this case, shot peening improves the contact fatigue resistance but surface modifications related to plastic deformation are enhanced in comparison to the Ni-free steel with a higher density. As a consequence, compressive residual stresses are smaller. The theoretical model was applied and the effect of the resistance to plastic deformation of the base material on the improvement of the contact fatigue resistance is discussed.