Abstract: From the Chinese drug Rabdosia amethystoides seven components were isolated, one of them is a new four ring diterpenoid to which the name Amethystoidin A is suggested. Its formula was found to be C₂₀H₂₈O₅, m. p. 260～262.5℃. By means of spectral methods combined with chemical analysis the chemical structure of Amethystoidin A was determined tobe ent-7α, 14β-dihydroxy-20α-hydroxymethyl-kaur-16-en-11, 15-dione。This compound was shown to have inhibitory action on experimental tumor in vivo and heptoma cells and staphylococcus aureus in vitro. The other components "B", "C" and "F" were identified to be ursolic acid, β-sitosterol and stearic acid respectively. The identification of other components is in progress.