The mechanism of wave-induced liquefaction and densification of a permeable seabed is theoretically and experimentally investigated. The results of this study are as follows:
　(1) The mechanism of the excess pore pressure generation due to ocean waves is quite different from that due to earthquakes.
　(2) The change of the effective overburden, which is evaluated as the difference between the pressure on the seabed and the fluctuating pore pressure in the seabed, causes the liquefacion, the expansion and the densification in the seabed continuously in one wave period.
　(3) The influence factors to wave-induced liquefaction and densification are theoretically and experimentally clarified.
　(4) Theoretical pore pressures calculated from the proposed theory taking
account of the influence factors compare well with measured pore pressures.
　(5) The result of the test suggests that the wave-induced liquefaction may cause the settlement of gravity type offshore structures and the rising of light-weight structures in the seabed.
　(6) Wave-induced liquefaction is thought to be one of the reason causing the scouring of the seabed.