Laboratary tests has been done mainly to determined the stabirity of GAMMER-EL block cover-layer for break-waters. The expression of stability with wave hight in deep water has been derived and correlated with the test data. In conjunction with the stability tests, data of wave run-up on GAMMER-EL block break-water and damping-action by GAMMER-EL block mounded break-water are obtained for tested break-water section and wave condition for stability.