The influence of atmospheric dustfall on the wall paintings and sculptures of Mogao Grottoes at Dunghuang has drawn much attention recently. In order to identify the origin of sand-dust, we examined the seasonal changes in the amount of dustfall, altitudinal distribution and mineralogical constituent. As a result, it was found that atmospheric dustfall originates from Mt. Mingsha. In addition, an examination of the amount of mold spores carried by dust showed that there were less inside the caves than outside and that their influence on the wall paintings is not great. Measures to prevent damage caused by dustfall is also discussed briefly in this paper.