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 degrees C. The presences of vacancy defects, represented by increment SHe+H parameter, induced by sequential irradiations was larger than the sum of defects, increment S-He parameter+ increment 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.