Abstract: Mechanism of formation of an anabranched estuary is discussed from the viewpoint of mechanics of sediment transport.It is suggested that the formation of the anabranched estuary is attributed to many factors,among which the bed-forming role of bedload is the most dominant one under suitable combination of boundary condition,flow and sediment transport.The diffusion and deceleration of flow due to abrupt change of estuarine boundary is necessry condition for the accumlation of bedloas and the development of estuarine bars.Morever,a very high clay content in suspended sediments which can ensure silting up and stability of estuarine bars also plays important role in the formation of anabranched estuaries.