Rare-earth ions of Nd 3 § and Er 3 § in nearly stoichiometric and MgO-doped LiNbO 3 crystals, respectively, have been investigated by employing an X-band electron spin resonance (ESR) spectrometer. The grown crystal was heated in Li-rich powder at ll00~ in order to make it nearly stoichiometric by the vapor transport equilibrium technique. Due to the fact that the ESR linewidth is much narrower in the stoichiometric crystal than in the congruent LiNbO3, we were able to determine the hyperfine constants of E43Nd and 145Nd at 4 K. By codoping MgO into LiNbO3, a new Er 3 § center has been observed with a different g-tensor. We propose that the new Er 3+ center in Mg-doped LiNbO 3 occupies the niobium site due to the local excessive Mg 2+ ion at the lithium site, whereas Nd 3+ and Er 3+ in congruent crystals reside at the lithium site. The proposal is consistent with the gvalue anisotropy.