From the leaves of Brassica juncea, an radical scavenging isorhamnetin 7-Ο-glucoside on peroxynitrite and 1,1-diphenyl-2-picrylhydrazyl (DPPH) was isolated and characterized based on the spectroscopic evidence. The compound showed the peroxynitrite and DPPH scavenging activities with $IC_{50}$ values of 2.07$pm$0.17 and 13.3 $mu$M, respectively. Penicillamine and L-ascorbic acid as positive control exhibited radical scavenging activities with $IC_{50}$ values of 3.17$pm$0.39 and 12.78 $mu$M, respectively.