In polycrystalline nickel-base superalloys, fatigue crack initiation from crystallographic facets is often found to occur within the coarser grains along or near twin boundaries in relation to the plastic slip band activity. Even if the thermomechanical processing route enables to obtain well-controlled grain size structure to avoid in-service failure, the understanding of the crack initiation mechanisms is still of paramount interest. In the present work, fatigue tests were performed over a wide range of amplitudes on a model microstructure. Two strain conditions were chosen in order to perform a statistical analysis of the first fatigue crack initiation sites. Microplasticity analyses near and on the cracks enabled to identify and discuss the crack initiation mechanisms.