Vacancy-type defects in ion implanted Si specimens were studied by a monoenergetic positron beam. Dominant defect species in 80-keV B+-implanted specimen were identified as vacancy clusters from their annealing behavior. For 150-keVP+-implanted specimen, oxygen atoms recoiled from a SiO2film grown on Si substrate were found to form stable complexes with vacancy-type defects in the Si substrate near the SiO2/Si interface. Since the migration of vacancy-type defects was blocked by such complexes, the final recovery of defects for the P+-implanted specimen occured at higher annealing temperatures than that for the B+-implanted one. For 2-MeVP+- implanted specimen, two types of oxygen-vacancy complexes were found to coexist even after annealing at 1200°C.