We investigated the stability of armor stones of mounds in composite breakwaters damaged and undamaged in spite of being under same condition. According the results, we proposed the fromulae of critical stadility of armor stones.
　The two fromulae were expressed by exponential function of d/H, the ratio of depth at the top of mound to wave height.
　One is available for the case the angle of wave incidence to breakwater is less than 45 degree and the other is in the cace that it is over than 45 degree.