Hot water soluble polysaccharide was extracted from Ulva fasciata. The isolated polysaccharide was purified using gel filtration chromatography on Sephacryl S-400. The chemical composition of the polysaccharide revealed the presence of sulphate. The anticoagulant activity of the purified polysaccharide was studied by prolongation of APTT Ex Vivo using rat model. The sulphated polysaccharide prolonged the APTT in a dose dependent manner and can be developed as a promising anticoagulant agent.