In this book we have reported the biological properties of five species of seaweeds collected from Rameshwaram coast, Tamil Nadu. The different extracts (methanol, hexane and acetone) of marine seaweeds Sargassum plagiophyllum, S. fusiforme, S. wightii, S. falciparum and Turbinaria ornata were screened for antibacterial activity against the 4 multidrug resistant bacterial strains. Among the different extracts T. ornata-hexane extract showed maximum activity against the Staphylococcus aureus, Klebsiella sp and Pseudomonas aeruginosa. Then, the aqueous extract of T. ornata was used for synthesis of silver nanoparticle. The nanoparticles were characterized using UV-Vis spectroscopy, FTIR, XRD, Zeta potential, DLS, FESEM and EDX. The larvicidal activity of hexane extract of T. ornata and silver nanoparticles of T. ornata was performed on two larval species of Aedes aegypti and Culex quinquefasciatus and the analysis showed that there was larvicidal effect. The AgNPs and hexane extract of T. ornata were tested for their anticancer properties using the lung cancer cell line (A549).