Wave action to breakwater and quay wall by fixed horizontal plates as shown in iig 4 and 5 are studied theoretically and experimentally Transmission and reflection coefficients for breakwater are calculated as shown m Fig 6(a)(b), which suggest that only fixing the water surface by rigid plate is effective to reduce the transmitted waves for appropriate wave conditions Pressure distributions to plates are calculated as shown in Fig 7 and total pressures are in Fig 8 Pressure distributions to norizontal plate and vertical wall of quay wall are shown in Fig 9 and 10, which show that the plate* makes the pressure distribution to vertical wall more uniform than the one without plate and also submerged plates of breakwater and quay wall make the pressure distributions smooth Wave action m case when the region under plates is filled by permeable material are also calculated The results suggest that the void of the material has the effect to elongate the plate and the fluid resistance acts as wave energy absorber.