The positron annihilation lifetime measurements have been performed to study the defects and radiation damage in the 1.45×1020n/cm2 and 3.10×1017n/cm2 neutron irradiated silicon as a function of annealing temperature from 343 K to 1073 K. The oxygen- vacancy （O- V） pairs were observed in both high and low dose neutron irradiated silicon samples and the quadrivacancies were detected during the annealing process in the high dose neutron irradiated silicon only. The measured short lifetime τ1 is a weighted average of lifetimes of positrons annihilated in bulk and those trapped at the O-V pairs. The long lifetime τ2 is attributed to positrons trapped at the divacancies or quadrivacancies.