Marine macroalgae contain many bioactive compounds, which have a wide range of applications.Ulvan (Sulfated heteropolysaccharide) extracted from Ulvalactuca and Ulvafasciata,have been characterized, antioxidant capability as well as its biological activity in affecting growth and metabolism of the tested microalga Chlorella vulgaris. The polysaccharides yield was about15% and 17 % on dry algal biomass basis respectively. The two Ulvasppcontainefour neutral sugars: rhamnose, glucose,galactose and xylose as estimated by HPLC. The FT-IR spectrum of ulvan showed bands corresponding to sulfate esters and uronic acids. Thermogravimetric analysis revieledthatulvan extracted from eachU. lactuca and U.fasciata is thermally stable before degradation at 169^oC and 245^oC respectively. Viscosity of U. faciataulvanismore viscous than that of U. lactuca (15.3 cps and 12.23 cps respectively).Antioxidant activity of ulvan is investigated through estimation of the reducing popwer. The different concentrations of ulvan has significant promoting effect on growth parameters of C. vulgaris (culture optical density, dry biomass, photosynthetic pigments, total soluble protein content and carbohydrate content) as well as non enzymatic antioxidants potential (represented by phenolic components content) and enzymatic antioxidants as Guaiacol peroxidase activity (GPX), Ascorbic acid peroxidase (APX) and Catalase activity (CAT). Ulvanenhances the microalgal growth via promoting antioxidants potential, consequentlyulvan can be considered as anpromising candidate material for agricultural, biomedical applications as well as biostimulant for mass production of microalgae.