Dissimilar metal electron beam welding with reduced activation ferritic/martensitic steel, F82H IEA heat, and SUS316L austenitic stainless steel was studied. Mechanical property evaluation at room temperature by bend test, tensile test, Vickers hardness measurement and charpy impact test, and evaluation of irradiation hardening by proton irradiation at 300°C up to 0.5 dpa were carried out. The mechanical properties of the dissimilar weld were improved by the optimization of the electron beam position in the welding (shifted 0.2 mm on 316L side) and the post-weld heat treatment (PWHT) (750°C x 1 hour). The improvement of the mechanical properties might be due to the fact that the weld metal consisted of the austenitic phase. Smaller irradiation hardening than 316L was observed in the weld metal of the F82H/316L dissimilar weld after PWHT at 750°C for 1 hour, where the electron beam was shifted 0.2 mm on 316L side, though the formation of voids and dislocation loops occurred in the grain matrix of the weld metal.