The ability of Sargassum siliquosum to prevent the initiation of free radicals to cause cellular damage was investigated in vitro. The plant was extracted exhaustively using methanol and partition using solvents of different polarities. Total phenolic contents (TPC) and flavonoid contents (TFC) were evaluated according to Folin-Ciocalteu and aluminium chloride colorimetric assays, respectively. TPC results showed significant difference in the mean gallic acid equivalent (GAE) (p<0.001) using the five fractions: water < (butanol-water = methanolwater) < hexane < dichloromethane (DCM), in which the mean GAE using DCM is significantly the highest with 69.03 mg/g GAE. TFC demonstrated a significant difference in the mean quercitin equivalent (p<0.001) in which the mean QE using dichloromethane is significantly the highest (65 mg/g) compared with other samples. Radical scavenging activities of fractions with the highest TPC and TFC were tested against OH, NO and Hâ‚‚Oâ‚‚. The strongest inhibition was demonstrated by DCM fraction against radical scavengers OH, NO and Hâ‚‚Oâ‚‚ in a dosedependent manner with IC50 value of 0.28, 0.29, and 2.27 mg/mL, respectively. These results clearly indicate the beneficial effect of S. siliquosum as antioxidant and anticancer agent being a free radical scavenger.