The Mekong river basin has a total area of about 795,000 km², and flows through 6 countries: China, Myanma, Laos, Thailand, Cambodia and Vietnam with the total length of the mainstream river of 4.800 km, and the annual flow of about 450 billion m³. The Mekong river is ranked 21^st for its total area, 12^th for its length and 8^th for its total annual flow. The Mekong river has a high biodiversity with about 1,300 aqua species, stands on the 2^nd place of annual fish production in the world, and the basin is where supplies food for about 300 million people. The Mekong river has a high potential of hydropower development with a total capacity of about 53,000 MW including 23,000 MW at the upper Mekong (in Lan Cang river) in China, 13,000 MW at the lower mainstream Mekong river, and more than 17,000 MW of hydropower potential in Mekong tributaries of 4 countries at lower Mekong basin. The agricultural and hydropower development plan in general and the plan to build 12 mainstream hydropower dams in particular may cause some negative impacts on hydrological condition, water quality, biodiversity, life and production activities of tens of million people in downstream countries. This paper presents some possible impact of upstream development scenarios on hydrological condition, environment and socio-economic development in the Cuu Long River Delta of Vietnam and points out the positive and negative impacts on the delta area and recommends some adaptive measures.