Krabi River Estuary, a ramsar site located in southern Thailand, is dominated by two major natural vegetation assemblages, i.e. mangrove forests and seagrass beds. According to the cluster development strategy, the cabinet has approved in principle to develop Krabi as the global marine tourism center and economic gateway of Asia. This study aimed to assess the economic value of the site, using market price and benefit transfer. The impact on economic activities with the 5% annual decline of mangrove forests was evaluated. The estimated annual use value of the site was $9.7 million for recreation and tourism. The economic value of mangrove forests was $758/ha. The net present value of mangrove forests was $73.1 million based on 7% discount rate and 15-year time line. An approximate impact of mangrove change follows the study of Ruitenbeek (1992). It was assumed that the impact on local direct resource extraction and loss in biodiversity would occur at the year after the depletion of the forest. The impact on the productivity of local fishery would occur in the fifth year and on tourism in the tenth year. Thus, value loss of mangrove at the annual rate of 5% in the net present term became $21.0 million or $2.3 million per year. The results imply that a development project that causes the same rate of mangrove destruction must generate a least an income of $2.3 million per year to be considered as an economically feasible project.