From the MeOH extract of the aerial part of Vietnamese Orthosiphon stamineus, five new isopimarane-type diterpenes [orthosiphols F-J (1-5)] and two new diterpenes [staminols A (6) and B (7)] with a novel carbon-framework, to which we proposed the name "staminane", and three new highly-oxygenated staminane-type diterpenes [staminolactones A (8) and B (9) and norstaminol A (10)1 were isolated. Moreover, staminolactone A (8) is 8,14-secostaminane-type and staminolactone B (9) is 13,14-secostaminane-type, while norstaminol A (10) is 14-norstaminen-type. Together with these new diterpenes, sixteen known compounds were also isolated and identified to be: 7,3',4'-tri-O-methylluteolin (11), eupatorin (12), sinensetin (13), 5-hydroxy-6,7,3',4'-tetramethoxyflavone (14), salvigenin (15), ladanein (16), tetramethylscutellarein (17), 6-hydroxy-5,7,4'-trimethoxyflavone (18), vomifoliol (19), aurantiamide acetate (20), rosmarinic acid (21), caffeic acid (22), oleanolic acid (23), ursolic acid (24), betulinic acid (25), and beta-sitosterol (26). All the isolated compounds were tested for their cytotoxicity towards highly liver metastatic murine colon 26-L5 carcinoma cells, and the new diterpenes, except for 4, and flavonoids (11, 12, 16, 18) showed cytotoxicity with an ED50 value between 10 and 90 microg/ml.