Abstract: After we successfully identified 40 constituents of essential oil of Schisandra sphenanthera Rehd, et Wils, collected from District of Wudu, Gansu Province of China, we have isolated four crystalline constituents from its seed. One of these, after recrystallized from ether-n-hexane, was a white plate crystal and melted at 160-161℃. By means of UV, IR, ¹HNMR,¹CNMR and MS, its structure was proposed as 3-kelo-9,10,19-cyclopropyl-lanost-24-en-26-oic acid. This compound has not been reported in literatures and named as Ganwuweizic acid.