Many literatures have focused on the design, fabrication and intrinsic damage of multilayer dielectric gratings (MLDGs). However, contamination may be a key factor of the MLDGs lifetime. In this paper, we compare the laser damages of the clean and contaminated MLDGs, and damage threshold of clean MLDGs is 3.12J/cm<sup>2</sup> . We obtain the curve of the critical particle size at various laser fluences, above which the MLDGs will be damaged. Intrinsic LIDT of clean MLDGs is larger than that of contaminated ones, so the cleanness control is a crucial problem for the MLDGs. The scanning electron microscope (SEM) morphologies of intrinsic and contaminant induced damages are different. The intrinsic damage is cold ablation, while contaminant induced damage is thermal ablation.