The characteristics of clay minerals in the soils derived from different parent materials in Hainan Island and their application in Chinese Soil Taxonomic,Classification are discussed in this paper.The results are as follows.In general,there are much kaolinite in the clay fraction of all soil samples,but the highest content and the worst crystaillinity of kaolinnite are present in the soils derived from basalt.The contents of hydromica are so much in the soils derived from schist or purple sandstone that they are as high as 40%-50% of the clay fraction.And the content of hydromica in the soil derived from limestone is also as much as 20%-37% of the clay fraction.Moreover,the hydromica belong to hydromuscovite with dioctahedral structure.The ratio of the content of goethite and that of hematite is (3-4):1 in the clay fraction of the soils of "udic" suborder derived from basalt and granite.13ut there is only goethite without hematite in the clay fraction of the soils of "perudic" suborder derived from granite on the mountain.This result proves that the types of iron oxide in soil response to the soil moisture conditions,which can be used as an index to distinguish the "udic" and "perudir" soil moisture regime in the Chinese Soil Taxonomic Classification.