The molecular structures and antioxidant activities of a novel flavonoid-type compound (lespedezaflavonone) from Lespedeza virgata were studied using density functional theory (DFT) with the B3LYP exchange correlation functional. The optimized geometries of neutral, radical cation, radical, and anion forms of lespedezaflavonone were obtained. Ring A was found to be responsible for the high activity of the flavonoids by an analysis of the character of the frontier molecular orbital, which was consistent with what was observed experimentally. Furthermore, it was noteworthy that ring A' was firstly found to be the important part for the potent antioxidant activity of lespedezaflavonone. To quantify the antioxidant activities, we determined the adiabatic ionization potential (IP, 509.0, 432.2 kJ·mol-1 for the neutral and anion forms, respectively), the homolytic O—H bond dissociation enthalpy (BDE, 347.3 kJ·mol-1) for lespedezaflavonone, the adiabatic electron affinity (EA, -620.6 kJ·mol-1) and the H-atom affinity (HA, -487.5 kJ·mol-1) for hydroxyl radical in aqueous solution. Our theoretical analysis shows that H-atomtransfer, stepwise electron-transfer-proton-transfer, and sequential proton-loss-electron-transfer mechanisms for lespedezaflavonone to scavenge hydroxyl radical may occur thermodynamically in parallel, while the last process is the most favorable. These findings are helpful for further study on the design of novel efficient flavonoid-type antioxidants, the structure-activity relationship and the antioxidant mechanismof flavonoid-type compounds.