Sc:Er:LiNbO3 crystals have been grown by Czochralski method. Their infrared (IR) transmission spectra were measured and discussed to investigate defect structure. Optical damage resistance of Sc:Er:LiNbO3 crystals was characterized by changes in the photoinduced birefringence as well as the transmitted beam pattern distortion method. It remarkably increases when the concentration of Sc2O3 exceeds a threshold. The optical damage resistance of Sc (3.0 mol%):Er:LiNbO3 is much higher than that of Er:LiNbO3. The threshold value of Sc3+ in Sc:Er:LiNbO3 crystals is below 1.714 mol %. Intrinsic and extrinsic defects are discussed to explain the threshold effect in Sc:Er:LiNbO3 crystals. (c) 2005 Elsevier B.V. All rights reserved.