The effect of point defects on the Curie temperature (T-c) of LiNbO3 (LN) was investigated by combining T-c measurements with an analysis of the defect structures of LN doped with impurities having various valences. The data show that T-c of congruent LN increases with the impurity concentration up to around 3 and 2 mol% for divalent and trivalent impurities, respectively, whereas it decreases continuously with increased concentrations of tetravalent impurities. These T-c variations were examined with respect to the defect structures of impurity-doped LN, which are expressed in the form chemical formulae using Kroger-Vink notation. The defect structures of divalent and trivalent impurity-doped LN are {[LiLix]1-5x-2y[NbLi....]x[MLi.]y[VLi]4x+y}[NbNbx][OOx]3 and {[LiLix]1-5x-3y[NbLi....]x[MLi..]y[VLi]4x+2y}[NbNbx][OOx]3, respectively (Nb-Li: Nb at Li sites; V-Li: vacancies at Li sites, M-Li: impurities at Li sites, and Li/Nb = the congruent ratio). The defect structure in the case of tetravalent impurities is {[LiLix]1-5x+y[NbLi....]x[VLi]4x-y}{[NbNbx]1-y[MNb]y}[OOx]3. Analyses of the defect structures indicated that the Nb-Li concentration decreases with divalent or trivalent impurity doping, which increases T-c. In contrast, the Nb-Li concentration increases with tetravalent impurity doping, which decreases T-c. In addition, the divalent or trivalent impurity concentrations at which the Nb-Li concentration becomes zero were found to correspond to the concentrations at which T-c is maximized, suggesting that T-c of LN depends on the Nb-Li concentration.