Recently, coastal disasters due to swells induced by heavy storms increase at any harbors. The wave overtopping rate of long period swell banded narrow spectra is different from that of wind wave because of the characteristics on the wave grouping and the wave height distribution. In the study, the model experiments were conducted to measure the wave overtopping rate of the long period swell on seawall and to propose the countermeasure to be able to be efficiently decreased the wave overtopping rate. As a result, it is possible to estimate the wave overtopping rate of the long period swell by applying the existing design method and to reduce the wave overtopping rate of long period swell by placing the permeable dethatched breakwater formed in concrete blocks in front of the seawall.