Long period waves with periods of 30 to 300s sometimes cause the large surging motion in mooring cargo vessels due to resonance. The interruption in cargo handling and break of mooring rope may be induced when the ship surging motion becomes remarkable. A new type sea wall composed of the double face slit caisson and rubble layer backward has been proposed in order to reduce the long period wave heights in harbor.
　A series of model experiments on efficiency of the long period wave absorbing sea wall were carried out. The experimental results demonstrated that the long period wave reflection coefficient became about 0.7 when the appropriate total length and diameter of rubbles were employed.
　A numerical model applying the direct fluid analysis method was developed to estimate the long period wave reflection profiles for the various type wave absorbing sea walls. The model was verified in comparison with the experimental results. The significant efficiency of long period wave absorbing sea wall in oblique incident wave field was numerically demonstrated.