Small angle neutron scattering experiments have been undertaken on the tempered martensitic steel EUROFER97 after proton irradiation at 250 and 350 °C and for doses of 0.3 and 1 dpa. It has been qualitatively shown that the scattering arises from distributions of irradiation-induced defects that peak below one nanometer in size (radius). The number density in the distribution increases with dose for each irradiation temperature. At the same dose, the density of the defects has been found much larger after irradiation at 250 °C than after 350 °C. The variation of ratio between the nuclear and magnetic scattering with dose suggests that the chemical composition of the nano-defects or helium content in the nano-bubbles is dose dependent.