An acid polysaccharide (SFP), was extracted from alga Sargassum fusiforme in hot water, was purified by ion exchange chromatography on DEAE-cellulose. The PC, chemical analysis, electrophoresis and IR of SFP indicated that it was a kind of alginate with a mol. wt. of 13,000 and a molar ratio of mannuronic acid to guluronic acid 2.75. Pharmacological tests showed that SFP could prolong the survival duration of mice suffering from ascitic Sarcoma 180 with a rate of life prolongation of 63.44%.