Myricetin, quercetin and kaempferol are polyphenols belong to the group of flavonoids. They are known for their many biological activities and particularly their strong ability to trap free radicals that cause nuisance to living organisms. In order to rationalize and compare the antioxidant activities of these molecules, DFT study was conducted in the gas phase, at B3LYP / 6-311G (d, p) and M05-2X / 6-311G (d, p). approximation levels. Calculations carried out relate to electronic affinity EA, ionization energy IP, energy gap (HOMO-LUMO), hardness (η), softness (S), electronegativity (χ), electrophilic index (ω) and energy parameters. Results of various calculations compared to those of trolox, molecule identified in our previous work as reference for study antioxidant properties of bioactive molecules have shown that: The three molecules are good antioxidants and could be effective to fight the oxidative attacks of living organisms; The hydroxyl groups of catechol group and C2 = C3 double bond are determinant for the antioxidant activity of the three molecules; Myricetin is the most antioxidant among the three molecules followed by quercetin; The radical ABTS°+ is more suitable for studying the antioxidant properties of molecules.