A known norcembranoidal diterpene, 5-episinuleptolide (1), along with a new analogue, 4 alpha-hydroxy-5-episinuleptolide (2), were isolated from a cultured-type soft coral Sinularia numerosa. The structures of 1 and 2 were elucidated on the basis of spectroscopic methods and by comparison of the data with those of the related metabolites. Cytotoxicity of metabolites 1 and 2 against a panel of tumor cells is also described. Compound 2 exhibited moderate cytotoxicity toward CCRF-CEM cells with an IC50 value 4.21 mu g/mL. Preliminary SAR (structure activity relationship) information was obtained from these two compounds.