Streptococcus pyogenes is associated with a wide range of infections and disease states, that infects children and adolescents causing sore throat. This study was carried out to evaluate the potential of exopolysaccharides of Aphanocapsa sp was tested for antibacterial activity against Streptococcus pyogenes ( Group A streptococcus) bacterial cultures using well diffusion method.Plackett- Burman design was applied on original BG11medium to determine the best conditions that effect on activity of exopolysaccharides of Aphanocapsasp against Streptococcus pyogenes. Plackett- Burman design was performed on original BG11medium by applied various nitrogen sources NaNO3and ammonium chloride. It was found that exopolysacchaaride extracted from Aphanocapsa sp had activity against Streptococcus pyogenes. Exopolysacchaaride extracted from alga that grown in Bg11medium supplemented with ammonium chloride had more effective against Streptococcus pyogenes than that obtained from Bg11medium supplemented with NaNO3.