This paper deal with the dune-rock which covered the seaward slope and top of coastal dune like a cap along the coast of Hainan Island, South China SeaEven though distribution along the coast is like beach-rock and ridge-rock, the coastal dune-rock is different from them in diagenetic environment and geomorphologic significance and can be distinguished by their sepe-rate sedimentary textures; structures and cements, A few identified types of low-Mg calcite cement that have been observed in the dune-rock are. Rim cement, meniscus cement, ghost structure, syntaxial rims, prefill cement and so on which reveal that the dune-rock is formed in vadose zone by leaching and evaporation of fresh water.Two conditions are necessary to form dune-rock along a dune coast.One is that at least 8% CaCO₃ is required for cementation(The contents of CaCO₃ in coastal dune sands here is more than 20%);And the other is that the layer of sand alternates frequently from wetting to drying cycles.According to the study, the coastal dune and dune-rock are formed at every stage during an eustasy so that the dune-rock can not be taken as a significant sign to the sea level changes,And the world wide distribution of dune-rock suggests that the correlation of dune-rock with definite climatic phases of the Holocene is not warranted.