Introduction: It's well documented that the oxidative damage, caused by reactive oxygen species (ROS) will lead to serious diseases such as cancer, coronary heart diseases, Diabetes etc. Pre-clinical pharmacological research with new marine compounds continued to be extremely active in recent years and Many studies around the world have been conducted to discover the biological activities of algae or to find compounds for various purposes such as pharmaceuticals, cosmetics, food preservatives, anti-deposition, and so on. Aim of study: this study held in order to evaluate the potentialities of seaweeds (brown algae) as a natural source of antioxidants Methods: Marine brown alga Iyengaria stellata collected from intertidal zone of Qeshm island in Persian Gulf, during summer and autumn 2016. After washing out the epiphytes and other unwanted fouling, the macroalgae biomass was dried and crushed to the powder.To determine free radical scavenging activity of the algae, DPPH (2, 2- Diphenyl-1-picrylhydrazyl) assay was used. In order to estimate Polyphenols, dry powder of I. stellate and lypolized water were used to extraction according Tariq et al. (2011). Results and discussion: The aqueous extract of I. stellate showed the higher (25.04%) antioxidant activity than ethanol extract (14.15%). The highest antioxidant activity was raised sharply till 180 minutes (64.25 %) and then it remained almost constant. In vitro antioxidant activity of both aqueous and ethanol extracts of I. stellate increased with increasing concentration indicating the dose dependency of these property. The ethanol extract showed the higher value of polyphenol content (2.48 mg % Gallic acid) and didn't show any correlation with antioxidant activity. Also, the dry algae contained higher concentrations of polyphenols in compared to water extracts.Both ethanol and lyophilized water extracts of I. stellate showed the significant antioxidant activity, wich increased with time of incubation. The present re
