The EtOAc fraction of Rhodiola rosea ethanolic extracts showed a strong antioxidant activity. Through activity-guided fractionation and purification, we isolated two flavonol glycosides, which were identified as the well known flavonoids, rhodionin (1) and rhodiosin (2). To compare their antioxidant activities, we used an authentic aglycone compound, herbacetin (3). Among the compounds tested, rhodiosin (2) exhibited strong antioxidant activity, with $IC_{50}$ values of 0.21 and $0.15;{mu}M$ against ${cdot}OH$ and ${cdot}O^-{_2}$, respectively. Rhodiosin (2) (100 mg/kg) reduced MDA content in the liver induced by irradiation when given prior to exposure of $gamma$-radiation.