The crude methanol extract of the fruits of Amomum tsao-ko (Zingiberaceae) showed cytotoxic activity. Bioactivity-guided separation led to the isolation of a diarylheptanoid, tsaokoarylone [7-(4-hydroxy-3-methoxyphenyl)-1-(4-hydroxyphenyl)-hepta-4E,6E-dien-3-one] (2). 2 showed cytotoxicity at 4.9 and 11.4 $mu$g/mL ($IC_{50}$) against human nonsmall cell lung cancer A549 and human melanoma SK-Mel-2, respectively, determined by SRB colorimetric method. During purification-(4-hydroxyphenyl)-4-hydroxyhexan-2-one (4) together with three known diarylheptanoids was also isolated. Their structures were determined from interpretation of spectroscopic data (IR, UV, MS, and NMR) and synthesis confirmed the structure of 2.