Methanol extract of Stellera chamaejasme L was assessed for antitumor activity by an antitumor active bioassay against murine leukemia P388 in vivo. The bioassay-directed separation of the extract furnished seven diterpene compounds (stellerarin, stelleramacrin, gnidimacrin, pimelea factor P₂ , subtoxin, huratoxin, simplexin ) and two biflavanone compounds (neochemaejasmin A and B). Among them, gnidimacrin, stellerarin and stelleramacrin (a novel compound) were found to have high antitumor and cytotoxie activities against P388, LI210 and K562 in vivo and in vitro. The results suggested that the diterpene compounds were the potent antitumor principles of Stellera chamaejasme L.