Two sets of HT-9 specimens were irradiated with 14 MeV nickel ions. One set of specimens were irradiated in the temperature range from 300 to 600°C and up to a peak dose level of 200 dpa and the other set of specimens were uniformly preimplanted with 100 appm of helium and then were irradiated at temperatures of 400, 500, and 600°C up to 60 dpa. The results indicated that there was virtually no void swelling at all in no helium specimens and even with 100 appm helium pre-implantation the maximum local swelling was found to be 0.1% in the specimens irradiated at 500°C to 60 dpa. This result further demonstrated the superior void swelling resistance of HT-9 ferritic steel and it is also concluded that free gas atoms are essential to the formation of cavities in heavy-ion irradiated HT-9. Other microstructural features resulting from the irradiation were also examined in detail, including dislocation loops and precipitates.