In:Tm:LiNbO<sub>3</sub> crystals were grown by the Czochralski technique with fixed concentrations of Tm<sub>2</sub>O<sub>3</sub> and differing
concentrations of In<sub>2</sub>O<sub>3</sub>. Their ultraviolet-visible absorption spectra were measured in order to investigate their defect
structures and their optical damage resistance was characterized by the transmission light spot distortion method. The
results show that the optical damage resistance of the In:Tm:LiNbO<sub>3</sub> crystals improves with the doping concentration of
In<sub>2</sub>O<sub>3</sub> increasing. The dependence of the optical damage resistance on the defect structure of In:Tm:LiNbO<sub>3</sub> crystals is
discussed in detail.