In the present paper, the stability of armor units for rubble mound foundations of composite breakwaters against the action of irregular waves has been investigated. The tests establish that irregular waves are more destructive than regular waves, when the regular wave height is set equal to the significant wave height. The action of regular waves becomes almost equivalent to that of irregular waves when the height of regular waves is set equal to 1.37 times of the significant wave height on the average.
　The stability number for armor stones and newly proposed concrete blocks are formulated on the basis of irregular wave tests, It is expressed by the function of
h'/H1/3 and k, here h' is the crest depth of the rubble mound foundation, H1/3 is the design significant wave height, and k is a parameter for the combined effects of the relative water depth and the relative berm width of the rubble mound foundation to the wavelength.
　The design weight of armor units can be calculated by the stability equation with the stability number. The application of the proposed method to the results of irregular wave tests demonstrates that the damage percent for the armor stones is at most 3.5% at the desgin condition.