The vacancy_dioxygen complex(VO 2 ) is one of the main defects formed i n fast neutron irradiated CZ_Si during annealing in the temperature range 400—500℃. In this defect,two oxygen atoms share a vacancy,each of which is bonded to two s ilicon neighbors.With the increase of the 889cm -1 (VO 2 ),two infrared absorption bands at 919.6 and 1006cm -1 will arise in neutron irradia ted CZ _Si after annealed in the temperature range 300—500℃.IR vibrational bands at 9 19.6 and 1006cm -1 can be assigned to the metastable defect (O-V-O)th at is composed of a VO(A center) and a neighboring interstitial oxygen(O i )atom.By prolonging the annealing time from 2h up to 10h or increasing the annealing temperature,the metastable defect(O-V-O)will be converted into V O 2 .During annealing in the temperature range 400—500℃,the main defe cts for med in the high dose(10 19 ) neutron irradiated CZ_Si is the multi_vac ancy type of defects and the formation of the VO 2 will be depressed.