A new phenylbutenoid dimer, (+/-)- trans-3-(4-hydroxy-3-methoxyphenyl)-4-[(E)-3,4-dimethoxystyryl]cyclohex-1-ene, was isolated from the rhizomes of Zingiber cassumunar along with the three known compounds, (+/-)- trans-3-(3,4-dimethoxyphenyl)-4-[(E)-3,4-dimethoxystyryl]cyclohex-1-ene, 4-(3,4-dimethoxyphenyl)but-1,3-diene, and 4-(2,4,5-trimethoxyphenyl)but-1,3-diene by bioassay-directed fractionation using the A549 human cancer cell line cytotoxicity assay. Structure of the new compound was elucidated by spectral analysis, including 1D and 2D NMR experiments.