In order to search for bioactive substances from marine organisms,the Taiwanese soft coral Clavularia inflata,collected along the coast of Green Island,Taiwan,was studied because of its significant cytotoxicity against NUGC-3 and HONE-1 cancer cell lines in EtOAc extract. Bioassay-guided fractionation resulted in the isolation of five diterpenoids,including four known compounds, ( 1R*, 12R* ) - dolabella- 4 (16), 7, 10- triene- 3, 13- dione (1),stolonidiol monoacetate (2), stolonidiol (3),clavutriolin (5),and one new compound,2- acetoxy- stolonidiol (4). Structures of these compounds were determined on the basis of spectroscopic evidence and chemical derivatization. The application of NMR techniques included 1D- and 2D- NMR,such as COSY, HMQC,HMBC and NOESY,etc. Compounds 1-5 are bicyclic dolabellane skeleton diterpenes. Compound 1 has two ketone groups at C-3 and C-13 positions,an exocyclic methylene at C-4 and two double bonds at C-7 and C-10 positions. Compounds 2-4 possess two epoxides at C-7,C-8,C-10,and C-11. Among them,compound 2 has an acetoxyl group at C-17,and a hydroxyl group at C-18. Compound 3 has two hydroxyl groups at C-17 and C-18 positions. Compound 4 has two hydroxyl groups at C-17 and C-18,and an acetoxyl group at C-2. Compound 5 contains three hydroxyl groups at C-8,C-17,and C-18,and a chlorine atom at C-7, and a epoxy moiety at C-10 and C-11 positions. The ctytotoxicity of these compounds and their derivatives against KB¡]human oral epidermoid carcinoma¡^,Hepa¡]human liver carcinoma¡^and Hela¡]human cervix epitheloid carcinoma¡^cancer cell lines were also measured. Compounds 1-3 and 5 exhibited significant cytotoxicity against KB cancer cell lines with ED50 values of 0.87,3.07, 1.35 and 0.36 mg/ml,respectively,meanwhile, compound 4 exhibited cytotoxicity against Hepa cancer cells with ED50 value of 1.20 mg/ml. However,the acetylated derivatives 6 and 7 were inactive against these cell lines.