A pentacyclic ent-kauranoid, nervonin A ( 1), with an unprecedented cyclobutane moiety in the structure, and nine other new ent-kaurane diterpenoids, nervonins B-J ( 2- 10), along with 10 known ones ( 11- 20), were isolated from Isodon nervosus. Their structures were elucidated by detailed spectroscopic analysis. All diterpenoids were assayed for their cytotoxicity against K562, A549, and HepG2 human cell lines. Compounds 2, 11, 16, 17, and 20 showed significant cytotoxicity.