Shaking table tests with large models of a composite breakwater composed of caissons and a rubble mound on sand layers are described. The tests were designed to study the dynamic water-structure-soil interaction and to discuss the dynamic stability of the breakwater when the liquefaction occurs in the sand layers during earthquakes. The sudden increase of residual displacements were measured on the model at the certain acceleration level. At this acceleration level, the excess pore pressure exceeded 50% of the effective confining stress in sand layers. The time histories of the horizontal acceleration computed by the seismic response analysis by FEM in which the nonlinear material behavior is modeled by the equivalent linear procedure agreed with the observed time histories. The safety factor calculated by the circular arc analysis in consideration of the seismic force and the excess pore pressure was less than 1.0 when the residual displacements of the models rapidly increased by an excitation of certain acceleration level.