The present study investigated the chemical composition and biological activities of the brown macroalga, Turbinaria ornata. The biochemical analysis of Turbinaria ornata indicated that there was a relative high protein, lipids, polysaccharides and carotenoids recorded during November; while the relative high soluble carbohydrate and chlorophylls were recorded during September. Heavy metals (Pb, Zn, Fe, Cd, Mn, Cu and Co) accumulated 6-22 times in T. ornata more than that in surrounding water with relative increases during November. Antimicrobial activities of ethanol extract of T. ornata as well as oleic acid, palmitic acid, fucoxanthin and fucosterol (extracted from the alga) were directly proportional to concentrations and varied with the growth stage of the alga as well as with the type of microb. These extracts showed high antibacterial activities particularly against Escherichia coli and Bacillus subtilis. Moreover the extracts exhibited high antifungal activities especially against Aspergillus niger and fusarium oxysporium. Scanning microscope showed that mixtures of oleic and palmitic acids caused plasmolysis and reduction in cell size of Escherichia coli, and malformation and swelling in conidiophore and spores of Aspergillus niger. On the other hand, cytotoxic activity using cultured mammalian cell lines indicated that the highest activity was recorded by fucoxanthin followed by fucosterol, and ethanol extract of T. ornata. These cytotoxic activities were directly proportional with the extract concentrations.