The seasonal variation of marine algal biomass and community were studied with nondestructive sampling and quadrat method during May 2006 to April 2008 in the intertidal and subtidal zone at Yokjido. As result, total 120 species, 13 Chlorophyta, 34 Phaeophyta, 71 Rhodophyta and 2 Spermatophyta were identified in this study. The major species which contributed significantly to the total biomass were Ulva pertusa, Undaria pinnatifida, Ecklonia stolonifera, Sargassum horneri, S. serratifolium, Gelidium amansii and Prionitis cornea. The mean biomass was 235.5 g wet wt m$^{-2}$ in intertidal, 1,038.9 g wet wt m$^{-2}$ in 1 m, 1,013.1 g wet wt m$^{-2}$ in 5 m, and 19.7 g wet wt m$^{-2}$ in 10 m depths of mean sea level. Seasonal biomass change in intertidal zone, the peak season was in spring, while the lowest was in autumn. The vertical distribution of marine vegetation was characterized by Ulva pertusa - Gelidium divaricatum - Gloiopeltis furcata - Sargassum thunbergii - Hizikia fusiformis at intertidal zone, and Caulerpa okamurae - Gelidium amansii - Chondrus ocellatus - Ecklonia stolonifera - Sargassum horneri - Gracilaria textorii at subtidal zone. These result indicates that the marine algal species and biomass of 1 m and 5 m depths in subtidal zone are greater than intertidal zone, and Ecklonia stolonifera and Sargassum spp. are the dominant species of Yokjido.