Coastal structures are constructed principally to protect the coast line. However, these structures also can act as artificial substrata for seaweeds. In particular, armor blocks, such as tetrapods, prove to be good algal substrata. Our field observations on the vegetation and standing crop of seaweed communities on armor blocks led us to the following conclusions: 1) Ecklonia cavacommunities grew on tetrapods that had been placed to coincide with the maturation period of E. cava. The communities have been maintained for more than four years. 2) The standing crop of seaweeds on an offshore breakwater composed of tetrapods placed seven years before was almost the same as that found under natural conditions. 3) Variations in roughened surfaces (pebbles or scores and grooves 3 or 46 mm wide) did not affect the growth of Sargassum spp. However, a greater number of Ecklonia stolonifera plants were observed attached to roughened surfaces compared to smooth surfaces.