IN THIS CONTEXT the brown alga Dictyota dichotoma isolated from Hurghada Sea-Shore, Red-Sea coast, Egypt (June 2015) was screened for its antimicrobial bioactivity. A laboratory experiment was conducted to test activity of ethanol extract of the brown alga Dictyota dichotoma against three types of Gram- positive and six of Gram-negative bacteria, and one type of fungi. Ethanol extract was selected for this study by using different concentrations (100, 200, 300, 400 mg) of alga extracts. Result revealed that the alga ethanolic extract exhibited high suppression activity against all tested pathogens. HPLC analysis of the chemical composition of algal extract was detect the presence of high amount of certain ingredients such as Flavonoids (Acacetin, Luteo.6-arabinose8-glucose, Naringin, Hespirtin, Rosmarinic and Naringenin by 3448.407, 453.209, 396.107, 387.429, 213.689 and 207.17 ppm, respectively) and Phenols (Benzoic, E-Vanillic, Pyrogallol, Catechein, Salycilic and Ellagic by 1075, 912, 785, 628 and 357 ppm, respectively). The present finding revealed that the tested alga can be used as possible natural source of active ingredients.