A method is proposed to estimate settlement of breakwater, for which mound foundation failure occurred during high wave storms. It is shown that this settlement depends on height ratio of high wave to critical wave, as well as the shape and period of high wave. Failure probability of mound foundation is calculated as large as 50-60%. However, the expected settlement of breakwater due to this kind of failure is estimated about 59-75cm. Such a degree of settlement may not damage a breakwater disastrously.