Using variable energy positron annihilation spectroscopy (VE-PAS) and deep-level transient spectroscopy (DLTS) techniques, a systematic investigation has been made on the annealing behavior of He-irradiation-induced defects in Si with different phosphorus and oxygen concentrations. Annealing temperature dependence of the VE-PAS spectra indicates that the annihilation of the Si defects proceeds through two annealing stages: (i) 150-250 degrees C and (ii) 250-450 degrees C. Taking into account the DLTS results, vacancy-phosphorus complexes and divacancies are found to be significantly annealed out in stages (i) and (ii), respectively. In particular, the annihilation velocity of the divacancies significantly depends on the oxygen concentration in the Si. Furthermore, at the end of stage (ii), the DLTS spectra also revealed that a new defect generates in Si with a relatively low oxygen concentration, which is considered to be associated with the high-order vacancy clusters formed by combination of some vacancies during the anneal-out of the divacancies. This energy level is so deep below the conduction band, that is likely to act as a thermally stable carrier recombination center.