The synthesis of Silver nanoparticles using Sargassum tenerrimum (powder or extract) is demonstrated here. The rapid formation of stable silver nanoparticles has been found using S. tenerrimum extract in aqueous medium at normal atmospheric condition. The antioxidant activity was determined by means of the Total phenolic content, Total antioxidant (TAA), De-oxyribose radical scavenging activity and DPPH radical scavenging test using different fractions of methanol extract. Result from these three methods indicated that the antioxidant activity of S. tenerrimum of methanol extract were time and concentration dependent. TPC exhibited higher activity in the methanolic extraction (6.26±0.45 mg GAE/g). TAA of methanolic extract were noted higher activity (0.77±0.17). The Deoxy- ribose radical scavenging activity (3.02±0.02% inhibition) of total methanolic extract and fractions of S. tenerrimum. The results showed that there was highest DPPH scavenging activity (1.53±0.44 %) hexane. In FT-IR analysis the major peaks were 3354.21 (Alcohol phenol O-H stretch) and 2972.31 (Alkyl C-H stretch).