Mao-Yan-Cao(Euphorbia Lunulata Bge.),a Chinese herb, has beneficial effect in the treatment of chronic bronchitis. Six crystalline compounds have been isolated from Mao-Yan-Cao by polyamide column chromatography and borax-ethyl acetate extracting method. Through spectroscopic analyses (UV, IR, NMR, MS) and the determination of physico-chemical constants, five of them have been identified as 3,5,7,4'-tetrahydroxyflavone (kaemperol),3,5,7,3',4'-pentahydroxyflavone(quercetin), kaompferol-3-mono-L-rhammoside, quercetin-3-mono-L-rhammoside and 6,7-dihy-droxycourmarin respectively. The sixth component is a new ooumarin derivative, named Maoyancaosu to which structure VI is tentatively assigned.