Single-beam (He+ or H+) and sequential-beam (He+/H+ or H+/He+) irradiation of reduced-activation martensitic steels at 450°C was carried out to investigate helium/hydrogen synergistic effects on the microstructure of the steels. After helium implantation, a very low density of helium bubbles was observed. No void was observed after hydrogen implantation. For the He+/H+ sequential-beam irradiation, the number density of bubbles increased rapidly at low hydrogen dose, and the average size of bubbles increased at higher hydrogen dose. Helium bubbles were also found in the H+/He+ sequential-beam irradiated specimen, but its swelling was smaller than that for the He+/H+ sequential conditions. It was concluded that the effect of hydrogen atoms on the nucleation and growth of bubbles may enhance the diffusion of helium-atom/helium-vacancy clusters. Hydrogen irradiation after helium irradiation can increase the nucleation and growth of helium bubbles, while helium bubbles may enhance the retention of hydrogen atoms to form He-H-vacancy complexes.