Kaurane diterpenoids and abietane diterpenoids are the major anti-tumor components in Tripterygium wilfordii (Celastraceae). Extraction of Tripterygium wilfordii afforded a new diterpenoid and twenty known diterpenoids belonging to the abietane and ent-kaurane families. The structures of all these compounds were characterized by spectroscopic techniques. All compounds were evaluated using the MTT method for in vitro cytotoxicity against six human cancer cell lines (HepG2, Hep3B, Bcap37, U251, MCF-7 and A549). Compounds 7 and 12 exhibited marked cytotoxicity against a six cell lines with IC50 values in the range 5.10-23.30μM. The structure-cytotoxic activity relationships of these diterpenoids are also discussed.