Seasonal and vertical variations of seaweed biomass were examined at Woejodo and Jusamdo of western sea, Korea from July 2006 to April 2007. Annual seaweed biomass was 198.27 g m$^{-2}$ in wet weight at Woejodo and 417.34 g m$^{-2}$ at Jusamdo, respectively and biomass of intertidal zone was greater than that of subtidal zone at Jusamdo sites. Seaweeds distributed vertically from mid intertidal to 5 m of subtidal zone at Woejodo and from high intertidal to 10m of subtidal zone at Jusamdo. Seaweed biomass and species number were maximal at lower intertidal zone (27 species, 365.43 g m$^{-2}$) of Woejodo and at mid intertidal zone (26 species, 684.18 g m$^{-2}$) of Jusamdo. Seasonal biomass varied from 136.73g m$^{-2}$ in autumn to 249.33 g m$^{-2}$ in winter at Woejodo and from 353.37 g m$^{-2}$ in autumn to 482.07 g m$^{-2}$ in summer at Jusamdo. Dominant species was Sargassum thunbergii showing highest annual biomass (Woejodo, 94.68 g m$^{-2}$ Jusamdo, 228.59 g m$^{-2}$) among all seaweeds and finding at various shore levels during the study period. Subdominant species were Corallina pilulifera and Gracilaria textorii at Woejodo, and were Sargassum fusiformis and Chondria crassicaulis at Jusamdo. Thus, we can conclude that Jusamdo shore is better place than Woejodo based on seaweed biomass and vertical distribution, and S. thunbergii is the representative species of the two islands.