The ultraviolet-visible and infrared absorption spectra of Fe 2O 3 and MgO co-doped lithium niobate (LiNbO 3) crystals were studied. It was found that some Fe 2+ ions were still in Li-sites though Fe 3+ ions changed their site occupation from Li-sites to Nb-sites while the doping concentration of MgO was above the threshold (the first threshold). Defect chemical analysis shows that the amount of Fe 2+ ions will decrease as the doping concentration of Mg increases, and when the doping concentration reaches a suitable value (the second threshold) all Fe 2+ ions in Li-sites will diminish. It is expected that LiNbO 3 crystal doped with the second threshold will have an excellent ability to resist laser induced optical damage. This phenomenon is called as double thresholds effect.