The vacancy_dioxygen complex(VO2) 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(VO2),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(Oi)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 O2.During annealing in the temperature range 400—500℃,the main defe cts for med in the high dose(1019) neutron irradiated CZ_Si is the multi_vac ancy type of defects and the formation of the VO2 will be depressed.