Sulfuretin (SFR), which is isolated from Rhus verniciflua , Toxicodendron vernicifluum, Dahlia, Bidens tripartite and Dipterx lacunifera , is one of the most important natural flavonoids. This compound is known to have numerous biological activities; among these, the antioxidant activity has not been thoroughly studied yet. In this study, the hydroperoxyl scavenging activity of SFR was examined by using density functional theory calculations. SFR is predicted to be an excellent HOO • scavenger in water at pH = 7.40 with k overall = 4.75 × 10 7 M −1 s −1 , principally due to an increase in the activity of the anionic form following the single-electron transfer mechanism. Consistently, the activity of the neutral form is more prominent in the non-polar environment with k overall = 1.79 × 10 4 M −1 s −1 following the formal hydrogen transfer mechanism. Thus, it is predicted that SFR exhibits better HOO • antiradical activity than typical antioxidants such as resveratrol, ascorbic acid or Trolox in the lipid medium. The hydroperoxyl radical scavenging of SFR in the aqueous solution is approximately 530 times faster than that of Trolox and similar to ascorbic acid or resveratrol. This suggests that SFR is a promising radical scavenger in physiological environments.