We extracted ulvan from the green seaweed Ulva reticulata collected from the Nha-Trang bay, central coast of Vietnam. The ulvan is composed of rhamnose, galactose, xylose, mannose, and glucose with molar ratio Rha:Gal:Xyl:Man:Glu = 1:0.1:0.11:0.01:0.21, uronic acid content of 19.3 wt %, and sulfate content of 14.3 wt %. Chemical structure determination using NMR and ESI-MS techniques showed that ulvan is composed of mainly two disaccharides beta-D-GlcAp(1-4-2)-alpha-L-Rhap and alpha-L-IdoAp(1 -> 4)-alpha-L-Rhap. Branching point is at O-2 of uronic acid. Sulfate groups were attached at C-2, C-3, and C-4 of the rhamnose residues.