Large scale coastal constructions cause tidal modification. Saemangeum dike, 33-km long, in mid-west of Korea and Isahaya dike, 7-km long, in west of Kyushu are the typical examples. Study of the tidal modification mechanisms on the both region is the fundamental step to understand and to predict the marine environmental change of the region. We use a three-dimensional numerical model to predict tidal change after the Saemangeum dike construction. To understand tidal change in the Ariake Sea after the Isahaya dike construction, a simple linear one-dimensional model is used.