A new type structure, named the high mound composite breakwater with slit-wall crown has been developed as a structure which will be applied for the protection of artificial islands as well as for reclaimed land. This new type structure is economical and suitable especially in a bay with a large tidal range.
　A series of small scale and large scale hydraulic model experiments are conducted for this new type high mound composite breakwaters. The wave force and the sliding stability of the upright section are investigated as well as the wave overtopping. Several types of modified superstructures were also tested.
　Major conclusions are as follows:
1) A high mound composite breakwater with a slit upright wall has high stability against severe wave actions.
2) Its wave overtopping was found to be smaller than that of rubble-mound-type seawalls.
3) The slit type superstructure with a slit upper beam was found to be most effective to reduce wave overtopping.