From the leaves of Rabdosia ternifolia (D. DON) HARA (Labiatae), a new diterpenoid, isodonoic acid (1) was isolated and the structure was established by spectroscopic and chemical evidence. Three known diterpenoids, isodonal (2), longikaurin E (3) and longikaurin A (4) were also isolated together with oleanolic acid and stearic acid.