Using a bioactivity-guided fractionation procedure, five cembranolides, 11-epi-sinulariolide acetate (1), 11-dehydrosinulariolide (2), sinulariolide (3), dihydrosinularin (4), and 3,4:8,11-bisepoxy-7-acetoxycembra-15(17)-en-1,12-olide (5), along with two nucleosides, 2'-deoxyadenosine and thymidine, were isolated from the Formosan soft coral Sinularia flexibilis. Moreover, 7,8-epoxy-11-epi-sinulariolide acetate (1a), 11-sinulariolide acetate (3a), dihydrosinulariolide (3b), 3,4:8,11-bisepoxy-7-hydroxycembra-15(17)-en-1,12-olide (3c), 11-acetoxyl-15(17)-dihydrosinulariolide (3d), 7,8-epoxy-11-sinulariolide acetate (3e), and 3,4:8,11-bisepoxy-7-hydroxycembra-15(17)-dihydro-1,12-olide (3f) were derived from compounds 1 and 3, respectively. These structures were deduced on the basis of physical and chemical evidence. Among them, 1a, 3d, 3e, and 3f are new cembranolide analogues. The structure of compound 1 was further confirmed by X-ray analysis. In addition, the isolated cembranolides and the analogues under went a cytotoxicity assay, and the structure-activity relationship (SAR) of these compounds was studied.