A new cytotoxic hydroperoxy-germacrane sesquiterpene named nephtheoxydiol (8) and several related sesquiterpenoids, ent-oplopanone (2), nephthenol (6), and nephthediol (9), were isolated from an Okinawan soft coral of Nephthea sp. (Nephtheidae), together with a new cadinanetype sesquiterpene named nephthene (17). Based on chemical reactions and physical data analyses, the absolute stereostructures of entt-oplopanone (2), nephthenol (6), nephtheoxydiol (8), nephthediol (9), and nephthene (17) have been determined as (+) -oplopanone (2), (4R, 7S)-germacra-1 (10) E, 5E-dien-4-ol (6), (1S, 4R, 7S)-1-hydroperoxygermacra-5E, 10 (15)-dien-4-ol (8), (1S, 4R, 7S) -germacra-5E, 10 (15) -diene-1, 4-diol (9), and (1S, 7R, 10S) -cadina-4 (14), 5-diene (17), respectively.