Alginates are well established as food additives and as encapsulation agents in biotechnology. Having a gelling ability, high viscosity and emulsion stabilizing properties, alginate and its derivatives are widely used in the food, cosmetics and pharmaceutical industries. Among alga diversity, the brown seaweed Sargassum genus is the largest natural seaweed resource of Vietnam, including about 50 species. Sargassum genus has considerable potential for use in remediation schemes and is enriched in polysaccharides. In this work, brown seaweed Sargassum swartzii collected at Nha Trang sea was treated with appropriate protocol to get sodium alginate. The yield was 26.7 percent of dry seaweed. 1 H-NMR spectroscopy was performed to elucidate the composition and block fractions of alginate. The results showed that homopolymeric guluronic acid block is dominated structure of extracted alginate (M/G=0.382) and average length of blocks of consecutive G monomers (nG=4).