In this work, the changes of irradiation defect in high dose (1.17 x 10(19) n cm(-3)) neutron-irradiated Czochralski silicon (CZ-Si) at different annealing temperatures was investigated using the Positron annihilation spectroscopy (PAS) and Fourier transform infrared spectrometer (FTIR) technique. It is found that divacancy V-2 and V-4 abound in irradiated CZ-Si. With the increase of the annealing temperature the monovacancy type defect (such as VO) is annihilated. The results of PAS and FTIR show that the formation of V-4 is enhanced when the annealing temperature ran up to 400-600 degrees C and with the FTIR absorption peak at the wave number of 829 cm(-1) (VO) disappearing, five absorption peaks appear at the wave number of 825cm(-1) (V2O2), 834 cm(-1) V3O2, 840cm(-1) V2O, 720 cm(-1) and 919 cm(-1). It can be concluded that these defect-impurity complexes prolong the lifetime of positrons. (c) 2004 Published by Elsevier B.V.