Background: Macroalgae can be viewed as a potential antioxidant and anti-inflammatory sources owing to their capability of producing compounds for its protection from environmental factors such as heat, pollution, stress, oxygen concentration, and UV radiations. Objective: To isolate major compounds which are mainly responsible for the pharmacological activity of brown alga under investigation, Sargassum sp. Materials and Methods: Algal material was air dried, extracted with a mixture of organic solvents, and fractionated with different adsorbents. The structures of obtained pure compounds were elucidated with different spectroscopic techniques, and two pure materials were tested for protection of DNA from damage, antioxidant, antitumor, and cytotoxicity. Results: Four pure compounds were obtained, of which fucosterol (1) and fucoxanthin (4) were tested; it was found that fucoxanthin has strong antioxidant and cytotoxicity against breast cancer (MCF-7) with IC 50 = 11.5 ΅g/ml. Conclusion: The naturally highly conjugated safe compound fucoxanthin could be used as antioxidant and as an antitumor compound.