The methanolic (MeOH) extract from the rhizomes of Zingiber cassumunar showed nitric oxide (NO) production inhibitory effects induced by lipopolysaccharide (LPS) in mouse peritoneal macrophages. From the MeOH extract, six new phenylbutanoids, phlains I—VI, were isolated together with 16 known constituents. The structures of new phenylbutanoids were determined on the basis of physicochemical and chemical evidence. In addition, the inhibitory effects of the principal constituents on the NO production were examined. Among them, phlain III (IC₅₀=24 μM), (E)-1-(3,4-dimethoxyphenyl)buta-1,3-diene (69 μM), (E)-1-(2,4,5-trimethoxyphenyl)buta-1,3-diene (83 μM), and cassumunaquinone 1 (47 μM) were found to show the inhibitory effects.