ABSTRACT Ultra-small testing technologies (USTTs), such as nanoindentation hardness tests and micropillar compression tests, can be potentially used to measure the mechanical properties of irradiated materials. However, the hardness and yield strengths of USTTs are different from those of standardized methods on bulk samples. In addition, irradiation-induced defects can affect the size dependence on the mechanical properties. This study examined A533B reactor pressure vessel steels before and after 5 MeV electron irradiation using USTTs. The depth dependence of the nanoindentation hardness was analyzed using the Nix-Gao model to evaluate the bulk-equivalent nanoindentation hardness and confirm its good linear relationship with the Vickers hardness. The compression yield strength was evaluated with a truncated square pyramid shape micro-pillars where the edge length (d) of the top surface and the height (L) of the micropillars were made similar: (d, L) = (1,2), (2,4), (3,6), (4,8), and (8,16) in μm. The size dependence of the yield stress in the micropillars has a minimum at d = 2 µm, which remained unaffected by electron beam irradiation up to 22 mdpa. The average yield stress of the d = 4 and 8 µm specimens was similar to that of the bulk tensile specimens.