The methylene chloride extracts of Formosan soft coral Sarcophyton crassocule Von. Marenzeller (collected at Green Island off Taiwan) were found to exhibit significant cytotoxicity against P-388, HT-29, and A-549 cancer lines. Chromatographic separation led to the isolation of three known cembrane diterpenoids, GN16-34 (1), GN16-35 (2), GN16-40 (3), and a new cembrane, GN16-62 (4). The methylene chloride and acetone extracts of Formosan soft coral Xenia puerto-galerae Roxas (collected at Green Island off Taiwan) were found to exhibit significant cytotoxicity against P-388 and A-549 cancer cell lines. Chromatographic separation resulted in the isolation of six new cadinane sesquiterpenes, GN44-28 (5), GN44-30 (6), GN44-44 (7), GN44-66 (8), GN44-173 (9), GN44-149 (10), and one known dicarbocyclic diterpenes with a bicyclic [4.3.1] ring system, GN44-199 (11). Compounds 3 and 4 exhibited significant cytotoxicity against P-388, HT-29, and A-549 cancer cell lines. Compounds 1 and 2 exhibited significant cytotoxicity against P-388 cancer cell lines. Compounds 5 and 7 exhibited significant cytotoxicity against A-549 cancer cell line. compound 10 exhibited significant cytotoxicity against P-388 and A-549 cancer cell lines.