Saemangeum is a shallow coastal area located at the southwestern part of Korea. It had very well developed intertidal zones with tidal range of about 5 m and the tidal currents of about 1 m/sec at the maximum, and riverine estuaries of the Keum, Mankyung and Donjin Rivers. Between 1992 and 2006, a series of 33km long tidal dykes resembling the inequality symbol with the vertex pointing west (<) and surrounding the Mankyung and Donjin riverine estuaries and intertidal zones was constructed. Using a finite volume coastal ocean model (FVCOM), which is based on triangular grid, the effects of the dykes on the tidal circulation in Saemangeum has been investigated by comparing cases with (“disturbed”) and without (“undisturbed”) the dykes. The model domain is about 100 km wide and 90 km long and the model is forced by tidal forcing at the open boundaries. Earlier studies suggest that the dykes weaken the tidal amplitudes along the open boundary of the current model domain by about 1 cm, but this effect is not considered. The tidal amplitude and phase from the model (“disturbed”) agree well with observations within 4 %. By comparing the “undisturbed” and “disturbed” we find that in front of the dykes the tidal amplitude was reduced by up to 7 cm. The effect of the dykes is stronger along the southern part of the dykes because it is parallel to the cophase line of the M2 tide, and more effective in blocking the propagation of the tidal energy. The northern branch is parallel to the undisturbed M2 tidal co-phase line and less effective in disturbing the tidal energy propagation. The tidal energy reflected by the southern branch accumulates around the Gogunsan island chains that are located west of the dykes.