Abstract: Mangrove peat is common in the peat flat environments on the Hainan lsland in the tropic zone. The he peat-forming plants are a tidal haluphilous mangrove community. This paper deals in detail with the characteristics of organic elcincnts and organic compositions in the mangrove peat. As compared with those in the fresh-water peat, the organic elementary compositions are characterized by higher oxygen and sulphur and lower carbon contents, and the humic acid compositions by higher fulvie acid contents in the mangrove peat. The organic compositions consist of higher contents of cellulose, seini-cellulose and lignin. This paper also discusses the variations in organic matter during peatificatiun processes. For example, carbon and oxygen contents tend to change anomalously in shallow peat and cellulose and lignin to turn into humic acid rapidly in response to the increase of the buried depth. A plenty of useful information can be provided from the study for exploring the mechanism for peatification on tidal flats.