PHYTOCHEMICAL screening was performed on six algal .genera belonging to three marine macroalgal divisions (Chlorophyta, Phaeophyta and Rhodophyta) collected from the coastal region of Red and Mediterranean coasts, Egypt during spring (2012). The antioxidant potential of the candidate seaweeds was evaluated by measuring the ferric reducing power (FRAP), total antioxidant capacity (TAC), total phenol and flavonoids, ascorbic acid as well as glutathione contents were determined. Significant individual differences of the biochemical parameters were recorded in all tested marine algae. The macroalgal phytochemical analysis showed quantitative variations in the total soluble and insoluble carbohydrates and proteins as well as the glycerol contents. The highest accumulation of carbohydrate was estimated in the members of Chlorophyta followed by Phaeophyta and Rhodophyta divisions. The protein content was higher in the species of Rhodophyta, moderate in Chlorophyta and the lowest was estimated in the members of Phaeophyta. In addition, the maximum glycerol content was recorded in Dictyota. Green algae exhibited greater levels of chlorophylls a and b. However, among all the studied three groups, the highest carotenoids were estimated in Padina followed by Dictyota; both belonging to Phaeophyta while the lowest carotenoid content was recorded in Gracelaria (Rhodophyta).The highest carbohydrate content was recorded in Enteromorpha where the lowest phenol was estimated. Results also revealed that, there was no relation between antioxidant activity and total phenols or flavonoid content. All the studied species are considered to be a rich source of antioxidants (ASA and GSH). Accordingly, these seaweed species have a high anti-oxidative potential and can provide dietary alternatives.