An alkaloid with a novel skeleton, oxazonigelladine (1), and four new dolabellane-type diterpenes, damasterpenes I(2) - IV(5), were isolated from the seeds of Nigella damascena. Oxazonigelladine has an isoxazolidinone partial structure bearing a direct bond between a nitrogen atom and an oxygen atom. This small and simple structure is quite rare in the nature. Surprisingly, despite the unstable bond between nitrogen and oxygen, oxazonigelladine was isolated with good stability. On the other hand, damasterpenes are dolabellane-type diterpenes with phenylacetyl, benzoyl, acetyl or nicotinoyl moieties. This report deals with the structure determination including the absolute configuration by X-ray crystallographic analysis and CD analysis. (C) 2017 Published by Elsevier Ltd.