In vitro antitumor activity of diterpenoids isolated from Rabdosia shikokiana var. occidentalis was determined against HeLa cells. Some of these compounds possess considerable activity against Ehrlich ascites carcinoma inoculated into mice. In particular, the epoxyketone 7 derived from shikoccin (1) showed potent activity. The in vivo activity of shikoccin (1) against six tumor lines was evaluated.