To search for useful antioxidants from plant materials, we investigated the antioxidative activity in the methanol extracts of 30 Chinese medicinal plants using DPPH method. The highest activity $(RC_{50}:;12;{mu}g)$ was showed in the methanol extract of Rosa rugosa, followed by Potentilla fruticosa $(14;{mu}g)$, P. fragarioides $(16;{mu}g)$. And Geum aleppicum $(18;{mu}g)$. From the leaves of R. rugosa, two antioxidative compounds were isolated by a bioassay guided purification and identified as isoquercitrin and ${eta}-glucogallin$ on the basis of $^1H-,;^{13}C-;NMR$, and FAB-MS data. ${eta}-Glucogallin$ is the first report in this plant. The DPPH radical scavenging activity of ${eta}-glucogallin;(RC_{50}:;8.5;{mu}g)$ was more effective than those of ${alpha}-tocopherol;(12;{mu}g)$ and BHA $(14;{mu}g)$.