Two new lignans, 4-methoxymagnaldehyde B (1) and coumanolignan (2), were isolated from the stem bark of Magnolia obovata, together with 11 known compounds (3—13). The structures of compounds 1 and 2 were determined to be 5-allyl-2-hydroxyphenyl-4-methoxy-3-cinnamic aldehyde (1) and 6-allyl-8-(5-allyl-2-hydroxyphenyl)coumarin (2) on the basis of spectroscopic and physicochemical analyses including 2D NMR and highresolution EI-MS. Compounds 1—8, 11, 12, and 13 were tested in vitro for their cytotoxic activities against the HeLa, A549, and HCT116 cancer cell lines. Among the compounds tested, compound 1 showed the strongest cytotoxic activity against the HCT116 cancer cell line, with an IC50 value of 1.3 mg/ml.