A regional ocean simulation system for tides, wind and ocean circulation in the seas bordering the Korean Peninsula and Japan is designed to cover an area that is broad in scope and size, yet provide a high degree of resolution in the area of scientific and engineering interests. With this simulation system, tidal distribution of the modeled regional seas and changes occurred in tidal regime due to coastal dikes as exemplified for Isahaya Bay in the Ariake Sea, Japan and Saemangeum Sea area, Korea are firstly estimated and further improvement and implementation of the simulation system are also discussed.