Recently, coastal disasters due to swell attacks induced by heavy storms and catastrophic typhoons increase at any ports and harbors. For example, the caissons of breakwater at the Fushikitoyama Harbor were damaged by the 14.2s swell in 2008. In general, in shallow water the wave height of swell becomes larger than that of wind wave under the effects of wave shoaling and wave refraction. The wave overtopping rate and wave pressure 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 and wave pressure on seawall of the long period swell and to propose the countermeasure to be able to be efficiently decreased the wave overtopping rate.
　The main conclusions of the paper are as follows.
1) It is possible to estimate the wave overtopping rate of the long period swell by applying the existing design method with attention to the length of wave period. And the wave overtopping rate tends to increase under the effects of the swell specific wave grouping.
2) The maximum wave pressure of long period swell becomes larger than that of wind wave above the still water level in the surf zone.
3) It is possible to reduce the wave overtopping rate of long period swell by placing the permeable dethatched breakwater formed in concrete blocks. However, the wave pressure is not efficiently decreased above the still water level.