A sesquiterpene ortho-naphtpoquinone was isolated from the 80% aqueous methanolic extract of root bark of Ulmus davidiana Planch whose stem and root bark have been used as an oriental medicine for the treatment of edema, mastitis, gastric cancer and inflammation. On the basis of spectral data obtained from UV-vis, IR, HR-EIMS and NMR spectrometry, including the pulse field gradient (PFG) HMQC and HMBC techniques, the structure of this compound was determined as 2,3-dihydro-3-hydroxy-3,6,9-trimethylnaphtho(1,8-b,c)pyran-7,8-dione which has been known as mansonone 1. This compound has been isolated only from the heart-wood of Mansonia altissima Chev, a Sterculiacea from West Africa. Thus, this compound was isolated for the second time from natural source in this study. This is the first report to present the carbon chemical shift assignment of mansonone I.