Macroalgae are important primary producers in marine ecosystem. They don't only play an important role as bioindicators but also provide economic resources for humans. Seasonal and vertical variations in seaweed species composition and biomass were examined to determine the ecological status of seaweed beds around Udo, near Jeju Island. We obtained samples at two sites in the high-intertidal to subtidal zones using the quadrat method between June 2010 and May 2011. A total of 262 species were collected, including 31 green, 61 brown, and 170 red algae. The composition of algal species revealed a decrease in species growing in cold water in comparison with the list 20 years ago. The macroalgal mean biomass (g wet wt m-2) was 3,476 g and 2,393 g from the two sites, respectively. Ecklonia cava had the greatest biomass at both sites. The seasonal dominant species by biomass at site 1 from the low-intertidal to 1-3 m depth of the subtidal zone was mostly comprised of thick-leathery form, such as Sargassum hemiphyllum, S. coreanum, and Ecklonia cava, whereas site 2 was comprised of the turf form, such as Chondrophycus intermedius, Chondracanthus intermedius, Dictyopteris prolifera, and Gelidium elegans. The current ecological status of the seaweed community in Udo is stable based on diversity and dominance indices.