The positron annihilation and perturbed angular correlation methods have been used to study the radiation damage and its thermal annealing behavior in the single crystal Si irradiated with fast neutrons of 1.45 ×1020/cm2 and 3.10×1017/cm2 and with 178W heavy ions of 5×1011/cm2

respectively.The measured positron lifetimes and quadrupole interaction frequencies show the existence of the monovacancysubstitutional oxygen complexes in the irradiated single crystal silicon.In silicon samples irradiated with 1.45×1020/cm2 neutron and heavy ions the combination of di-vacancies into quadri-vacancies has been observed by both methods.