The Saemangeum dike, 33-km long enclosing 40,100 ha, is being constructed in mid-west estuarine area of Korea. The dike, construction started in Nov. 1991, was completely closed in April 2006 except two sluice gates. Marine environments in Saemangeum area have been changing drastically since the dike closure. The change of tidal regime and the water-quality near the Saemangeum area are calculated using numerical models to interpret marine environmental changes. Results of numerical models and field observations show that the tidal current in the Saemangeum area is drastically reduced after the dike closure. It causes the deterioration of the water quality inside and outside area of the dike. When fresher water inside the dike flows out through two sluice gates, it can cause a shock on the outer marine environment. An integrated monitoring and modeling study is conducting to predict long-term variation and to mitigate the impact on the marine environment.