Bioassay-directed fractionation of the CH(2)Cl(2)-MeOH extract of Clavularia viridis collected in Taiwan has afforded seven new prostanoids, designated as 4-deacetoxyl-12-O-deacetylclavulone I (1), 4-deacetoxyl-12-O-deacetylclavulone II (2), bromovulone II (4), iodovulone II (5), 4-deacetoxyl-12-O-deacetylclavulone III (6), bromovulone III (7), and iodovulone III (8), in addition to seven known prostanoids (clavulones I, II, III, 7-acetoxy-7,8-dihydroiodovulone, chlorovulones II, III, and 4-deacetoxylclavulone II (3, claviridenone E)). The structures of compounds 1-8 were determined on the basis of 1D and 2D NMR techniques including COSY, HSQC, and HMBC experiments. Pharmacological study revealed that bromovulone III (7) and chlorovulone II exhibited the most promising cytotoxicity against human prostate (PC-3) and colon (HT29) cancer cells.