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The macro-invertebrates on natural (rock) and artificial (cement) boulders were compared along the southern Caspian Sea and the effect of structural features of boulders (i.e. orientation, facing, surface complexity, the degree of exposure to the wave action) on macro-invertebrate communities were investigated. Ten locations with rock walls in the southern Caspian Sea were investigated in which the isolated boulders of natural and artificial types with similar dimensions were haphazardly selected for sampling from their macro-invertebrate communities. A total of 59120 individuals of macro-invertebrates were counted being represented by 5 species from 3 phyla (i.e., Arthropoda, Annelida, Mollusca). The average taxonomic richness on natural boulders was significantly higher than that of artificial ones, but no significant difference was found in average density of macro-invertebrates between two boulder types. A significant difference in density and taxonomic richness of macro-invertebrates was found among different spatial orientations (i.e. vertical, sloped, horizontal) within and between boulder types. Density and taxonomic richness of macro-invertebrates on natural and artificial boulders were not significantly affected by other structural features including facing, surface complexity and the degree of exposure to the wave action. The results indicate that artificial boulders may mimic natural boulders only for density of macro-invertebrates.