A bioassay-guided fractionation of the stem bark extract of Eucommia ulmoides Oliver (Eucommiaceae) led to the isolation of three iridoid constituents, genipin (1), geniposide (3), geniposidic acid (4) as well as $({pm})-guaiacylglycerol$ (2) and fatty acid mixtures as active ingredients of the extract responsible for the antitumoral property. The EtOAc soluble part and BuOH soluble part of the extract demonstrated a potent inhibition on the proliferation of cultured human tumor cells such as A549 (non small cell lung), SK-OV-3 (ovary), SK-MEL-2 (melanoma), XF498 (central nerve system) and HCT-15 (colon) in vitro, whereas the remaining water soluble part exhibited a poor inhibition. The intensive investigation of the EtOAc soluble part and BuOH soluble part of the extract yielded genipin, guaiacylglycerol, geniposide, geniposidic acid and large amounts of fatty acid mixtures as active components.