Abstract: In order to research into the optical damage in iron-doped lithium niobate crystals,we investigated the decay behavior of the optical damage in iron-doped lithium niobate crystals.The dark decay time of the optical damage in thin crystal sheets was found to far exceed that in the thick crystal sheets.The OH-spectra and Raman spectra were performed in order to clarify the underlying mechanism.In conclusion,the thickness of samples greatly influences the optical damage.