Sargassum myriocystum was selected to evaluate for reactive oxygen radical scavenging capacities and antioxidant activities using commonly accepted assays. After shade drying, marine seaweeds were powdered and extracts were prepared by cold and Soxhlet methods using various organic solvents. Crude powder materials were used for determination of bio-chemical constituents like protein, carbohydrate and phenolic contents. It was found that phenolic contents constituted 22 mg gallic acid equivalents/ gm of dry plant materials. Methanol/chloroform extracts of Soxhlet showed highest reducing power, total antioxidant and hydroxyl scavenging activity. 80 to 120% inhibition of hydroxyl radical quenching activity was observed and demonstrated by all Soxhlet derived extracts, except petroleum ether. Hence, from the results we conclude presence of novel compound(s) in the crude extract of (methanol: chloroform) S. myriocystum is responsible for its hydroxyl radical scavenging activity. © 2011 Academic Journals.