Abstract: Eighty-five subaqueous large sand dunes were found in five areas on the Mekong river subaqueous delta by the side scan sonar investigations after the winter monsoon in 2003 and 2004. The duns are 1.4 m to 13.2 m high and 72 m to 672 m long. According to Ashley’s classification, they are large and very large subaqueous dunes. These dunes are generally asymmetric in shape with steep slopes dipping to W-WS. Water circulation induced by winter monsoon together with the reverse Kuroshio (branch) current in the South China Sea is the major driving force to the formation of dunes. Observation shows that the surface current velocity of winter monsoon circulation in the Cam Ranh Bay ranges from 1m/s to 1.4 m/s and the subaqueous dunes were formed during the low sea-level period of the last deglaciation. At present they are still migrating along the west side of the South China Sea under the winter circulation from N to W-SW and the migration velocity is in a dimension of 2.78×10^-5 m/s.>