The microstructures of reduced-activation ferritic/martensitic steels, 9Cr-1MoVNb, 9Cr-1MoVNb-2Ni, 9Cr-2WVTa and 9Cr-2WVTa doped with 2% Ni, irradiated at 400 °C up to 12 dpa in the high flux isotope reactor, were investigated by transmission electron microscopy. The cavity number density of Ni-doped steels was higher than that of Ni-undoped steels due to the higher concentration of helium. There was no difference of cavity number density between the steels tempered at 700 and 750 °C, but the mean size of the cavities in the steels tempered at 750 °C was larger than that tempered at 700 °C. There was a tendency for the number density of loops in Ni-doped steels to be higher than in Ni-undoped steels. In addition, the mean size of loops in the steels tempered at 750 °C was larger than for those tempered at 700 °C, while there was not much difference of number density between them. In the steels doped with Ni, irradiation-produced precipitates, identified as M 6C( η)-type carbide and M 2X phase, were found in the 9Cr-2VWTa-2Ni steel and 9Cr-1MoVNb-2Ni, respectively. Irradiation of Ni-doped steels showed the effect of helium on cavity nucleation, however, the effect of helium on dislocation structure and precipitation was not made clear.