Here, we investigated the irradiation defect in reduced activation ferritic/martensitic steels by slow positron beam. Three ion-irradiation experiments were carried out: (i) He²⁺ irradiation, (ii) H⁺ irradiation and (iii) He²⁺ irradiation followed by H⁺ irradiation, at temperature 450 °C. The presences of vacancy defects, represented by ∆S_He+H parameter, induced by sequential irradiations was larger than the sum of defects, ∆S_He parameter + ∆S_H parameter, caused by single He ions and single H ions. The synergistic effect of He and H was confirmed clearly from the perspective of positron annihilation spectroscopy.