Austenitic stainless steels, mainly grades 316L and 304L, are used to manufacture primary piping components in Pressurized Water Reactor (PWR) nuclear power plants. The thermal fluctuations and expansions to which these components may be subjected during operation induce loading comparable to low-cycle fatigue (LCF) under strain control. These steels are also exposed to a potentially corrosive environment, the primary PWR environment (light water at 325°C and 150 bar). Environmentally-assisted fatigue can lead to a reduction in the fatigue life of these materials. While the impact of the environment on fatigue life is well documented, the role of the environment on the mechanisms of crack initiation and propagation is much less understood. Several studies have shown that transgranular cracks initiation mode is predominant under LFC conditions. Micro-cracks formation occurs at the surface in 3D dislocations structures, known as Persistent Slip Bands (PSBs). These structures are the result of the deformation localization in the grains during cyclic loading. The irreversibility of dislocations movements leads to the formation of a surface relief composed of extrusions and intrusions. Previous work has also identified a critical extrusion height of about 250 nm above which a crack can be observed in air at 20 °C. The studies also showed that the environment has an impact on the initiation kinetics, which explains the fatigue life reduction in the primary PWR environment compared with the fatigue life in vacuum and air. The greater reduction in the primary PWR environment is explained by chemical interactions between the environment and the metal in zones with increased plastic activity, the PSBs. The aim of the present study is to understand the effect of the environment (vacuum, air, light water) on fatigue crack initiation mechanisms. Local and multimodal characterizations will therefore be carried out at initiation sites from the PSBs formation to the micropropagation in the three environments. To this end, interrupted or multi-interrupted LCF tests will be carried out in vacuum, air and primary PWR environment. Test conditions, especially the prescribed strain amplitude and strain rate, are set to observe a significant effect on fatigue life in the presence of the primary PWR environment. Following these tests, several observations will be made at the scale of one or more grains (SEM, EBSD), then at the scale of PSBs and associated dislocations structures (FIB-cut, TEM). The observations made will also enable analysis of variations in local chemical composition (EDX, EFTEM, EELS).