Abstract: Radiation damage in Nb produced by reactor neutrons of 10~(23) n/cm~2 fluence is simulated by 81 MeV carbon ion irradiation. The induced radiation damage and its thermal annealing behaviour are studied by both time differential perturbed angular correlation and positron annihilation techniques. The results show the existence of mono- and divacancies in BCC metal Nb following the 81 MeV carbon irradiation with a total dose of 2.5×10~(16)cm~(-2). As annealing temperature increases, divacancies are dissociated and total concentration of monovacancies decreases. After annealing at 1058 K, defects in Nb resulted from radiation damage completely disappear.