[en] Dislocation loops induced by single (He or H) ion beam and sequential (H/He or He/H) ion beam irradiation in reduced activation martensitic steels were investigated with transmission electron microscopy (TEM). Specimens were irradiated with 30 keV He⁺ and 18 keV H⁺ respectively under three different doses at 450 °C. Dislocation loops with mean size of 41.3 nm were observed under single He ion beam irradiated to 0.1 dpa. Sequential ion irradiation could induce larger dislocation loops than the case of single ion irradiation. It was interesting that mean size of dislocation loops induced by He/H sequential ion irradiation was larger than that induced by H/He sequential ion irradiation at the same dose. Possible explanations were presented and synergistic effects of He and H on dislocation loops were discussed.