The ethanol extract of Stellera chamaejasme roots exhibited significant bioactivity against Peries rapae. In the course of a search for the active compounds, S-(+)-1,5-diphenyl-3-hydroxy-1-pentanone (1) was isolated and this compound has never been found before in S. chamaejasme as far as we known. The structure was established on the basis of various spectral data, including IR, UV, 1H NMR, 13C NMR, EIMS, and chemical synthesis. Mortality against P. rapae was also determined.