Fucoidan extracted from seaweed, particularly Sargassum angustifolium, is considered asnatural antioxidant agent.The present survey was aimed to assessthe effect of fucoidan extracted from S. angustifolium on oxidative stability of butter produced from sour cream. Fucoidan was extracted from S. angustifolium samples. Prepared fucoidan powdered were weighted in 0.05, 0.1, 0.3, 0.5 percent and added to butter samples. All samples were then kept in the refrigerator up to 60 days. The DPPH radical scavenging effects of extracted fucoidan and also acidic and peroxide values of butter samples were evaluated. Fucoidan (100 µg/mL) had the uppermost DPPH radical scavenging effect (56.46±5.27%), while that of 12.50 µg/mL had the lowermost (34.75±3.26%). Acidic value of butter samples had been increased during the storage period (P<0.05). Butter samples of the control group had the highest acidic value in the 60^th day of the storage (0.40±0.033 mg KOH/g butter), while those treated with the 0.5% fucoidan had the lowest (0.17±0.013 mg KOH/g butter). Peroxide value was increased from day 0 to day 40 of storage period and then decreased. Butter samples of the control group had the highest peroxide value in the 40^th day of the storage (20.71±1.65 meq/kg butter), while those treated with the 0.5% fucoidan had the lowest (0.17±0.013 meq/kg butter). Using 0.5% fucoidan caused lower increase in the levels of peroxide and acidic values of butter samples during the 60 days storage period. The antioxidant effects fucoidan extracted from S. angustifolium seaweed is dose-dependent.