A reduced-activation martensitic steel was implanted with helium up to 580 at. ppm by using 36 MeV α-beam between 353 and 423 K along with displacement damage up to 0.226 dpa. The implantation-induced increase in ductile-brittle transition temperature (DBTT) was estimated to be 98 K for the standard charpy V-notched (CVN) specimen implanted with 580 at. ppm He, through the conversion of small punch (SP) test results by an empirical relationship. It is clarified from comparison with neutron irradiation data that the increase in DBTT as well as implantation-induced hardening is interpreted simply in terms of displacement damage, suggesting that there is no significant effect of helium on both the irradiation hardening and the fracture toughness of the steel. No fracture mode change by the helium implantation was observed in the SP tests, showing a complete cleavage fracture mode in the lower shelf energy region.