Long waves with periods of 1-5 minutes sometimes terminate cargo handling and cause breaking of mooring ropes in Japanese several ports. Large surging and swaying motion induced by resonance between long period waves and mooring system of vessel mainly cause such damages. The amplitude of these long waves are enlarged by local harbor oscillation. Breakwaters are usually constructed ageinst wind waves agitation in harbor, however, they have some effect to prevent long period waves from penetrating into harbors. More effective vountermeasures against such long period waves are inevitable for safe cargo handling in to long period waves. The present paper experimentally investigates the effects of breakwaters located at the entrance of a port to reduce the height of long period waves. The main conclusions are as follows :
(1)The long period wave height is reduced by offshore breakwater.
(2)Reducing the openings of ports is also effective against penetration of long period waves.