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Oxide dispersion strengthened (ODS) alloys containing high-density nano-oxides show excellent mechanical properties and irradiation tolerance. However, ODS alloys are often ultrafine-grained materials since the grain size of matrix is located between 100 and 1000 nm. Here, nanocrystalline and ultrafine-grained ODS alloys with different grain sizes (50 nm and 300 nm) were prepared by mechanical alloying and high-pressure consolidation techniques, which show exceptional thermal stability. To understand their irradiation response, the two alloys were irradiated with 400 keV helium ions to a fluence of 1 × 10¹⁷He⁺ /cm² at 350 ℃, 400 ℃ and 450 ℃. The results show that the size and volume fraction of helium bubbles in nanocrystalline ODS alloys increase when the irradiation temperature increases from 350 ℃ to 450 ℃. Grain boundaries (GBs) play a key role in bubble formation and evolution. We identified …