We studied the organisms attached to Sargassum beds in Gamak Bay from November 2007 to June 2008. The water temperature ranged from 3.3 to 28.7℃and salinity from 30.6 to 33.8 psu. Eighteen species 143,432 ind./m2 were sampled during study periods. The biomass of the Sargassum beds ranged from a highest of 196.08 g dw/m2 in March to a lowest of 0 g dw /m2 in July. The major dominant species were Barleeia angustata (134,430 ind./m2), Caprellidae sp.(2,443 ind./m2) and Gammaridae sp.(4,201 ind./m2). The ecological indices of diversity, evenness, and richness were 0.11-0.86, 0.04-0.39, and 0.74-1.06, respectively.