By functional B3LYP of the DFT, and in three more and more widened bases set 6-311G, 6-311G (d, p) and 6-311 ++ G (d, p), a theoretical study of the antioxidant properties of two isomeric flavonols (fisetin and kaempferol) was carried out. The electronics parameters as gap HOMO-LUMO, potential of ionization and electronic affinity were calculated, and also thermodynamics parameters as enthalpies of single electron transfert, of proton transfer and of hydrogen atomic elimination. The electronic calculated parameters allowed to estimate the powers electron donors and electron acceptors of molecules and to confirm their classifications in the literature according to the oxidizing power. On the basis of the calculated thermodynamics parameters, three various mechanisms of elimination of the peroxide radical (O2°) were explored for each studied molecules:  Electron elimination followed by proton elimination by the molecule, then trapping of free radical;  Proton elimination followed by electron elimination, then trapping of the free radical;  Elimination of hydrogen atom by homolytic rupture of OH bond, then trapping of the free radical. The results of the various calculations have confirmed the classification of molecules according to the antioxidant power, as presented in the literature, and have identified the mechanism through the elimination of atomic hydrogen by homolytic bond breaking, the most likely for the removal of a peroxide radical by each of the two molecules. The theoretical results also confirm that the most important sites of demonstration of the antioxidant activity of both flavonols are especially their catechol hydroxyl groups.