The herbs of Euphorbia supina (Euphorbiaceae) have been used to treat hemorrhage, chronic bronchitis, hepatitis, jaundice, diarrhea, gastritis, and hemorrhoids as a medicinal herb. This work is aimed to qualitatively and quantitatively analyze the polyphenols with peroxynitrite-scavenging activities. The eight compounds: gallic acid, methyl gallate, avicularin, astragalin, juglanin, isoquercitrin 6"-gallate, astragalin 6"-gallate, and ellagic acid, were isolated from E. supina and used for HPLC analysis and peroxynitrite ($ONOO^-$)-scavenging assay. Simultaneous analysis of the eight compounds was performed on MeOH extract and its fractions. The contents in MeOH extract and peroxynitrite-scavenging activities of the dimer of gallic acid, ellagic acid (15.64 mg/g; $IC_{50}$ $0.89{mu}M$), and two galloylated flavonoid glycosides, astragalin 6"-gallate (13.72 mg/g; $IC_{50}$ $1.43{mu}M$) and isoquercitrin 6"-gallate (16.99 mg/g; $IC_{50}$ $1.75{mu}M$),were high, compared to other compounds. The legendary uses of E. supina could be attributed to the high content of polyphenols, particularly ellagic acid, isoquercitrin 6"-gallate, and astragalin 6"-gallate as active principles.