10.22376/ijpbs.2019.10.1.p1-12 Volume 9 Issue 3 2018 (July-September) Sea weeds have fascinated pharmaceutical industries for their proactive molecules which have high demand as antioxidants. The marine environment in which seaweeds survive facilitates the synthesis of diverse secondary metabolites with interesting biological activities either as nutraceuticals or therapeutic applications. Similarly, they contain different types of inorganic and organic components which can be used for human health such as polyphenols, carotenoids, tocopherols, terpenoids, ascorbate, alkaloids etc. In the present study, the edible marine algae lessThan i greaterThan Kappaphycus alvarezii lessThan /i greaterThan lessThan b greaterThan lessThan /b greaterThan and lessThan i greaterThan Hypnea musciformis lessThan /i greaterThan , were evaluated for their phytochemicals followed by quantification, purification of terpenoids and evaluation of their lessThan i greaterThan in vitro lessThan /i greaterThan antioxidant activity in terms of 1,1-diphenyl-2-picrylhydrazyl(DPPH),2,2'-azinobis 3ethylbenzthiazoline-6-sulphonic acid (ABTS), ferric reducing antioxidant power (FRAP), reducing power, superoxide, hydroxyl radical scavenging and hydrogen peroxide radical assay, the metal chelating activity. Qualitative analysis revealed a pool of phytochemicals such as carbohydrates, proteins, alkaloids, glycosides, flavonoids, steroids, saponins, phlobatannins and polyphenolics. The quantified data of lessThan i greaterThan K. alvarezii lessThan /i greaterThan were carbohydrate – 38.28 mg/g, protein - 1.06 mg/g and fat - 1.02 mg /g of dry wt., whereas lessThan i greaterThan H. musciformis lessThan /i greaterThan revealed 18.02, 1.2 and 0.934 mg/g of dry wt. respectively. Polyphenols represent the major secondary metabolite and especially the terpenoids compounds. The terpenes composition from the red algae lessThan i greaterThan H. musciformis lessThan /i greaterThan and lessThan i greaterThan K. alvarezii lessThan /i greaterThan  were identified and estimated by using Gas Chromatography- Mass Spectrum. The crude methanolic extract of lessThan i greaterThan H. musciformis lessThan /i greaterThan and lessThan i greaterThan K. alvarezii lessThan /i greaterThan were purified by Column Chromatography and the fractions were subjected to GC-MS analysis. Purified terpenoids showed remarkable antioxidant activity in terms of scavenging hydroxyl radicals in a dose-dependent manner. Further, the extract exhibited strong inhibition on the H lessThan sub greaterThan 2 lessThan /sub greaterThan O lessThan sub greaterThan 2 lessThan /sub greaterThan and DPPH scavenging assay. The IC lessThan sub greaterThan 50 lessThan /sub greaterThan values were significant and comparable with synthetic antioxidants like ascorbate and quercetin. Thus, the selected seaweeds may be included in daily dietaries to combat energy malnutrition and micronutrient deficiencies. SUMAYYA S AND MURUGAN K appaphycus alvarezii, Hypnea musciformis, Column chromatography, GC-MS analysis, Antioxidant activity 68-73