The Saemangeum tidal flat with an area of approximately 233 km2 is one of the biggest estuarine tidal flats on the west coast of Korea. Because of its location in the estuary of Mangyeong and Dongjin Rivers, the tidal flat receives large amount of nutrients and contains diverse fauna and flora. A 33-km long sea dyke, enclosing a coastal zone of 40,100 ha, is being constructed to reclaim tidal flat in the Saemangeum area. The dyke construction radically changes the local tidal current regime and estuarine circulation. These have an effect on sedimentary environments and ecosystem in the tidal flat. The comparison of topography and surface sediments in summer 2004 with those in summer 1988 before the dyke construction were that elevation increased with maximum 2 meter and mean grain sizes were fining at Sandong, Ganghwal and Uma tidal flats. Sedimentary facies of two cores from Gwanghwal tidal flat revealed homogeneous layers in the upper part, suggested rapid deposition after the dyke construction. The sedimentation rate of GW 6 core by using 210Pb analysis was calculated to be 6.1 cm/yr after dyke construction.