Ions offer the opportunity to irradiate a specimen within a few hours up to doses equivalent to those reached in several years with neutrons in a nuclear reactor, and this, without activating the sample. Unlike neutrons, the implantation depth of heavy ions is limited to a few microns. Due to this shallow damage, micro-mechanical tests are particularly suited to study the evolution of mechanical properties under irradiation. Among them, nanoindentation permits to evaluate the variation in hardness, and micropillar compression tests provide information on the evolution of the yield strength via the critical resolved shear stress. Compared to nanoindentation, micropillar compression tests are more complex to perform but simplify data analysis, as compression is assumed to be uniaxial. These micro-mechanical tests exhibit size effects, meaning measured values depend on the probed volume. While the Nix–Gao model [1] permits to estimate bulk hardness from nanoindentation in ion-irradiated specimens, obtaining bulk properties from micropillar compression test remains challenging. Differences in defect density and nature bias direct yield strength comparisons between annealed and irradiated steels of the same pillar size, as size effects vary between these conditions. In this study, complementary nanoindentation and micropillar compression tests at various penetration depths and pillar sizes were performed on an annealed 304L steel. Linear relationships between bulk hardness and yield strength are well established for both unirradiated and irradiated steels [2]. By combining this proportionality with extrapolated bulk hardness from nanoindentation, size effects in micropillar compression tests performed on austenitic single crystals were highlighted. This study aimed to establish a criterion to evaluate hardening under irradiation based on a similar size effect rather than a similar pillar size. REFERENCES [1] W.D. Nix, H. Gao, Indentation size effects in crystalline materials: a law for strain gradient plasticity, Journal of the Mechanics and Physics of Solids (1998) 411–425. [2] J.T. Busby, M.C. Hash, G.S. Was, The relationship between hardness and yield stress in irradiated austenitic and ferritic steels, Journal of Nuclear Materials 336 (2005) 267–278.