We investigate the influence of vacuum organic contaminations onlaser-induced damage threshold (LIDT) of optical coatings.Anti-reflective (AR) coatings at 1064 nm made byTa2O5/SiO2 are deposited by the ion beamsputtering method. The LIDTs of AR coatings are measured in vacuum andin atmosphere, respectively. It is exhibited that contaminations invacuum are easily to be absorbed onto optical surface because of lowerpressure, and they become origins of damage, resulting in the decreaseof LIDT from 24.5 J/cm2 in air to 15.7 J/cm2 in vacuum. TheLIDT of coatings in vacuum has is slightly changed compared with the valuein atmosphere after the organic contaminations are wiped off. Theseresults indicate that organic contaminations are the main reason of theLIDT decrease in vacuum. Additionally, damage morphologies havedistinct changes from vacuum to atmosphere because of the differencesbetween the residual stress and thermal decomposability of filmymaterials.