Three new lignans, 4'-methoxymagndialdehyde ( 1), 4'-methoxymagnaldehyde B ( 2), and 4'-methoxymagnaldehyde E ( 3), were isolated from hexane- and EtOAc-soluble fractions of the stem bark of Magnolia officinalis, together with eight known compounds ( 4- 11). The structures of compounds 1- 3 were determined on the basis of spectroscopic and physicochemical data analysis. Compounds 1- 11 were tested in vitro for their cytotoxic activity against the K562, HeLa, and A549 cancer cell lines. Among the compounds tested, compound 1 showed the most potent cytotoxic activity against these cancer cell lines, with IC50 values of 3.9, 1.5, and 3.7 microg/mL, respectively.