A new sesquiterpene (Z)-7-acetoxy-methyl-11-methyl-3-methylenedodeca-1,6,10-triene (AMDT) was isolated and identified from the methanol extract of the hairy root culture of Artemisia annua. The structure of AMDT was determined based on the analysis of spectroscopic data, notably of the 2D NMR spectra. This new compound showed cytotoxicity against human tumor cell lines 95-D and HeLa with IC 50 values of 27.08 and 20.12 μmol/L, respectively.