The microstructures of reduced-activation ferritic/martensitic steels, 9Cr-2WVTa and 9Cr-2WVTa doped with 2% Ni, irradiated at 400oC up to 12 dpa in the High Flux Isotope Reactor (HFIR), were investigated by transmission electron microscopy. Specimens were tempered at two different temperatures in order to investigate the effects of tempering on microstructural evolution during irradiation. Before irradiation, the lath width of Ni-doped 9Cr-2WVTa was somewhat narrower and the dislocation density tended to be higher compared with 9Cr-2WVTa. Dislocation density of specimens tempered at 750°C was lower than that tempered at 700°C. In all steels, precipitates on grain and/or lath boundaries were mainly M23C6, and there were a few TaC along dislocations in the matrices. Irradiation-induced cavities were observed in all the steels. The cavity number density of the Ni-doped 9Cr-2WVTa was higher than that of 9Cr-2WVTa due to the higher concentration of helium; however, swelling in each steels was < 0.1% because cavity sizes were so small. There was no difference of cavity number density between the steels tempered at 700°C 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. Irradiation-induced a0<100> and (a0/2)<111> type dislocation loops were observed in all steels; number density and mean diameter of a0<100> type loops was higher and larger than that of (a0/2)<111> type loops. There was a tendency for the number density of loops in Ni-doped 9Cr-2WVTa to be slightly higher than that in 9Cr-2WVTa. 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 M6C(η)-type carbide, were found in the matrices. In this experiment, the change in tensile properties and the δDBTT of the 9Cr-2VWTa-2Ni was greater than for 9Cr-2WVTa. The microstructural evidence suggests that these differences in mechanical behavior are related to the formation of radiation-induced precipitates and cavities in the Ni-doped alloy; further analysis of the data using barrier hardening models is in progress.