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 high-resolution 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 IC(50) value of 1.3 microg/ml.