Tungsten-potassium (WK) alloy with ultrafine/fine grains and nano-K bubbles is fabricated through spark plasma sintering (SPS) and rolling process. In this study, 3-MeV W2+ion irradiation with a tandem accelerator is adopted to sim-ulate the displacement damage caused by neutrons. As the depth of irradiation damage layer is limited to only 500 nm, the hardening behaviors of WK alloy and ITER (Inteational Thermonuclear Experimental Reactor)-W under several damage levels are investigated through Bercovich tip nanoindentation test and other morphological characterizations. The indenter size effect (ISE), soft substrate effect (SSE), and damage gradient effect (DGE) are found to infl uence the measurement of nano-hardness. Few or no pop-ins in irradiated samples are observed while visible pop-in events take place in unirradiated metals. Extensive pile-up with different morphology features around the indentation exists in both WK and ITER-W. The WK shows a smaller hardness increment than ITER-W under the same condition of displacement damage. This study provides beneficial information for WK alloy serving as a promising plasma facing materials (PFMs) candidate.