Naturally occurring substances which strongly scavenge peroxynitrite(ONOO-) are evaluated to be beneficial for cardiovascular, diabetic, hypercholesterolemic and obese disease. To search for bioactive substances with peroxynitritescavenging activity from the young leaves of Heracleum moellendorffii(Umbelliferae) used for a mountainous vegetable, the experiment on peroxynitrite-scavenging assay, phytochemical isolation and HPLC analysis were undertaken. Activity-guided fractionation led to the separation of astragalin and hyperoside which were identified by physicochemical and spectroscopic data. The $IC_{50}$ values of astragalin and hyperoside were shown as $5.80{pm}0.62$ and $0.560{pm}0.26;{mu}M$, respectively, in the assay. The HPLC analysis led to the quantitative determination of astragalin and hyperoside by $30.0{pm}0.01$ and $17.0{pm}0.02;mg/g$ dried weight, respectively.