A highly potent antitumor sesquiterpene O-naphthoquionone has been isolated from Ulmus davidiana, which has been traditionally used as the folk medicine. The structure of this compound was established on the basis of spectral data obtained from UV, IR, MS, and NMR spectrometry, and determined as mansonone E. This compound showed potent antitumor activity on in vivo assay, hollow fiber and xenograft assay.