In, Er double-doped LiNbO3 crystals have been grown. The X-ray fluorescence measurements were used to measure the concentration of Er3+ and In3+ ions in the crystals. The ultraviolet-visible absorption spectra were measured and discussed in terms of the defect structure of the In:Er:LiNbO3 crystals. The optical damage resistance of In:Er:LiNbO3 crystals were characterized by the transmitted beam pattern distortion method. The optical damage resistance of In (3.0mol%):Er:LiNbO3 was more than three orders of magnitude higher than that of Er:LiNbO3. The defect structures were used in this paper to explain the optical damage resistance in the In:Er:LiNbO3 crystals. (C) 2004 Published by Elsevier B.V.