The study was conducted on four species of seaweeds, Sargassum feldmannii collected from KhanhHoa province and Sargassum gracile, Sargassum mcclurei, Sargassum polycystum collected from NinhThuan province. Seaweeds were extracted in two different solvents which were water and ethanol. Theextracts obtained from water extraction at temperature of 50°C ranged from 10.68%-18.98% andethanol extraction at temperature of 40-50°C was 13.48%-17.86%. The total polyphenol content ofwater extraction was lower than ethanol extraction, the highest in S. gracile (90.01 ± 1.01 mg/g). Thefree radical scavenging activity of water extraction was lower than ethanol extraction, the highest in S.gracile (71.23 ± 0.39%) and lowest in S. feldmannii (38.11 ± 0.12%). Comparing the antioxidantcapacity of the extracts with synthetic antioxidant additives: BHA, BHT, ascorbic acid and tocopherol,the strongest resistance rate was 151.39% with BHT in S. gracile, All of water or ethanol extractions ofS. feldmannii, S. mcclurei and S. polycystum had resistance rate in the range of 40-90%.Keywords: antioxidant activity, brown seaweed, Sargassum, total polyphenol content