A new (3) and three known germacrane-type sesquiterpenoids were isolated from the chloroform-soluble fraction of the methanolic extract of the bark of Magnolia kobus (Magnoliaceae) through repeated silica gel and Sephadex LH-20 column chromatography. Their chemical structures were elucidated as costunolide (1), parthenolide (2), isobisparthenolidine (3), and bisparthenolidine (4) by spectroscopic analysis. Compounds 1-4 exhibited cytotoxicity against human A549, SK-OV-3, SK-MEL-2, and HCT15 tumor cells.