Marine algal flora of three uninhabited islands of Sipidongpado, located in the middle western coast of Korea were investigated in regarding the floristic composition and vertical distribution at intertidal zone during July 2008. A total of 27 marine algal species (3 green, 8 brown and 16 red algae) was identified from the three islands. Among them, 15 species occurred at Byeongpungdo, 17 at Keunssangseom, 22 at Deungdaeseom. Eleven species were found at all three islands: Ulva pertusa, Ulva conglobata, Hizikia fusiformis, Sargassum thunbergii, Gelidium divaricatum, Corallina pilulifera, Caulacanthus ustulatus, Chondria crassicaulis, Polysiphonia morrowii, Symphyocladia latiuscula and Symphyocladia linearis. The total biomass of macroalgae in July at Deungdaeseom was 2,387.2 gm−2. Dominant species was S. thunbergii showing highest biomass (1,512.1 gm−2) in the intertidal zones at Deungdaeseom. Thus we can conclude that S. thunbergii is the representative species of this island.