The difference of annealing behaviors of vacancy-oxygen complex (VO) in varied dose neutron irradiated Czochralski silicon: (S1 5×1017 n/cm3 and S2 1.07×1019 n/cm3) were studied. The results show that the VO is one of the main defects formed in neutron irradiated Czochralski silicon (CZ-Si). In this defect,oxygen atom shares a vacancy,it is bonded to two silicon neighbors. Annealed at 200 ℃,divacancies are trapped by interstitial oxygen(Oi) to form V2O (840 cm-1). With the decrease of the 829 cm-1 (VO) three infrared absorption bands at 825 cm-1 (V2O2),834 cm-1 (V2O3) and 840 cm-1 (V2O) will rise after annealed at temperature range of 200-500 ℃. After annealed at 450-500 ℃ the main absorption bands in S1 sample are 834 cm-1,825 cm-1 and 889 cm-1 (VO2),in S2 is 825 cm-1. Annealing of A-center in varied neutron irradiated CZ-Si is suggested to consist of two processes. The first is due to trapping of VO by Oi in low dose neutron irradiated CZ-Si (S1) and the second is due to capture the wandering vacancy by VO,etc,in high dose neutron irradiated CZ-Si (S2),the VO2 plays an important role in the annealing of A-center. With the increase of the irradiation dose,the annealing behavior of A-center is changed.