Gallic acid is a ubiquitous phenolic compound, widely distributed in the plant kingdom and frequently found in the human diet. In this study, we explored its antioxidant potential through the determination of HOMO and LUMO energies, O–H bond dissociation enthalpy, ionization potential, electron affinity, and spin density distribution using the density functional theory. We have interpreted the radical scavenging capacity of gallic acid with the help of its structure and noted that the higher antioxidant potential of gallic acid was primarily due to the formation of radicals after abstraction of H atoms from –OH groups. In addition, we have found that the H atom transfer mechanism is preferable over single-electron transfer mechanism and the sequential proton loss-electron transfer mechanism for free radical scavenging capacity. On the basis of calculated results, it was also concluded that the antioxidant activity of gallic acid was due to the 3-OH and 4-OH groups, because of their low BDE values. Moreover, gallic acid is predicted to be among the best antioxidants identified so far. These theoretical researches will be helpful to the development for the antioxidant compounds.