The decoction of stem bark of Magnolia obovata Thunb. (Magnoliaceae) has been applied frequently for the treatment of stomachic disorders in Chinese Traditional medicine, As a part of the search for potent antitumor substances from natural resources, we have found that the stem bark extract of M. obovata had exhibited a significant cytotoxicity against cultured human tumor cell lines in vitro. Thus, a phytochemical investigation has been undertaken for the isolation of active constituents from the plant by way of a cytotoxicityguided fractionation procedure combined with the SRB (sulforhodamine-B) method (1) for cytotoxicity evaluation, which finally led to the isolation of six active constituents (1 —6), responsible for the cytotoxicity against five cultured human tumor cell lines which were currently used in the National Cancer Institute (USA) for the in vitro anti-cancerdrug screening program, i.e., A549 (non-small cell lung), SK-OV-3 (ovary), SK-MEL-2 (melanoma), XF498 (central nerve system) and HCT-15 (colon), in vitro. Here, we report briefly the isolation and the cytotoxicty of active components of the plant.