Water-soluble sulfated polysaccharides extracted from Ulva intestinalis and fractionated using DEAE Sepharose fast flow column to identify their molecular properties and macrophage cells stimulating activities. Crude and fractions (F1 and F2) were formed of neutral sugars (58.7-74.7%), sulfates (6.2-24.5%), uronic acids (4.9-5.9%) and proteins (3.2-10.4%). Different levels of sugar constituents including rhamnose (30.1-39.1%), glucose (39.0-48.4%), galactose (0.0-15.8%), xylose (8.5-11.3) and arabinose (0.0-5.1%). The molecular weight (Mw) of crude and fractionated polysaccharides ranged from 87.1 × 103 to 194.1 × 103 (g/mol). Crude polysaccharides were not toxic to RAW264.7 cells and fractions induced cell proliferation. Fraction F1 stimulated RAW264.7 cells to release considerable amounts of nitric oxide, IL-1β, TNF-α, IL-6, IL-10 and IL-12 cytokines. The main backbone of the most immunostimulating polysaccharide (F1) was consisted of mixed linkages of (1 → 2)-linked rhamnose and (1 → 2)-linked glucose residues.