Keywords: 
• Nansha islands /
• coral reef /
• halimeda /
• sedimentary facies

Abstract: Halimeda, a genus of calcified green alga (Chlorophyta) of warm seas, is an important producer of carbonate sediment on coral reef areas of Nansha Islands, South China Sea. This study analysed the modern sedimentary characteristic of Halimeda in 52 clastic samples from the 4 geo morphic-sedimentary zones of 8 atolls. Among the 4 zones, i. e., reef front slope, reef flat, lagoon slope and logoon basin floor, Halimeda is the richest in lagoon basin floor and constitutes averagely 32.66% of the sediment, and it ranges from 16% to 75%. Observation under microscope indicates that in the sediment of lagoon basin floor there are most well preserved Halimeda segments and they are deposited in situ. In the sedi-ment of lagoon slope, Halimeda ranges from 8% to 16%, with an average of 9.22%. In reef flat, Hal-imeda composition is low, and ranges from 1%to8%, with an average of 6.06%. From the present da-ta, the contribution of Halimeda to the reef front carbonate producti on is small. The analysed results are consistent with the field investigation. Halimeda of Nansha Islands can grow on any zone of coral reef seither in sand or on solid substrata, the optimum habitat is well closed, big lagoon area, deep wa-ter, weak dynamic, sandy lagoon basin floor, where it develops well in erect. This paper concludes that the sedimentary characteristic of Halimeda on coral reefs of Nansha Islands are in line with its ecologic distributi on. Coral reef drilling research shows that the paleoenvironment setting of the Cenozoic is com-parable with that of recent Halimeda, so the characteristic can be the bases of analysing sedimentary fa-cies of ancient and modern coral reef sediment.