Positron annihilation measurements show that the radiation defects and secondary defects in neutron-transmutation-doped (NTD) Si irradiated by two neutron doses can be removed at 600-650°C. The concentration and annealing behavior of V-type and V2 defects are related to the neutron doses. V4 appears at 150°C and 450°C - 550°C in NTD Si irradiated by higher neutron dose.