Gnidimacrin, a diterpene compound, isolated from the methanol extract of Stellera chamaejasme L., showed significant antitumor activities against mouse leukemia P- 388 and L-1210 in vivo, such as Lewis lung carcinoma, B-16 melanoma and Colon cancer 26. It showed ILSs of 40%, 49% and 41% at the dosages of 0.01-0.02 mg/kg ip, respectively. Gnidimacrin strongly inhibited cell proliferation of human cancer cell lines such as leukemia K562, stomach cancers Kato-lll, MKN-28, MKN-45, and mouse leukemia L-1210 by the MTT assay and colony forming assay in vitro. The IC₅₀ of gnidimacrin was 0.007 - 0.00012 μg/ml. It is concluded that gnidimacrin is the principal antitumor element in Steilera chemaejasme L. with strong antitumor activities.