We study in this paper the temperature dependence of the OH- absorption band in LiNbO3 crystals with different Hf-doping concentrations. It is found that the shoulder at 3500 cm(-1) appears in the doping level of 2.6 mol%, becomes more evident at 3 and 4 mol%, and then gradually faints when the doping concentration increases. This result is explained by the non-monotonic change of Nb vacancy density in the three stages of the defect evolution involved in the Hf incorporation of LN lattice. Moreover, the absorption difference analysis of the OH- bands is consistent with this explanation and reveals that the mobility of H+ ions may be reduced by the strong trapping of the highly electronegative Nb vacancies.