Abstract: The identification of four components of Rabdosia macrophylla was accomplished, three of them were shown to be identical with the known Amethystoidin A, ursolic acid and Enmenol. The 4th. is a new B-seco-ent-kaurene diterpenoid, to which the name Rabdophyllin G is suggested. Spectroscopic investigation and chemical conversion into dihydroisodonal (Ⅲ) and dihydroisodonal derivative (Ⅳ) established the structure of Rabdophyllin G. It exhibited significant inhibitory action against Ehrlich ascites carcinoma in mice.