Helium implantation was performed by cyclotron with a beam of 36MeV-α particles at temperature below 429 K using a beam energy degrader to obtain homogeneous distribution of helium in a reduced activation 9Cr-2W martensitic steel up to a concentration of 120 at.ppm. The ductile-brittle transition temperature (DBTT) was evaluated by small punch (SP) tests in which the disk specimens of 3 mm diameter with 0.22 mm thickness were deformed by bulge mode. The shift in the DBTT (SP-ΔDBTT) and the increase in Vickers hardness (ΔHv) caused by helium implantation were measured to be 26 K and 67 kg/mm2, respectively, which were estimated to be 65 K of standard CVN-ΔDBTT and 104 MPa of increase in the yield stress (Δσy) from empirical relationships between them. The enhancement of irradiation hardening by helium was never recognized and no change in the fracture mode at low temperatures was observed by the helium implantation. It is considered that the shift in the DBTT is mainly associated with irradiation hardening by displacement damage.