DFT method was employed to locate transition state for H-atom transfer from phenol by methyl radical and methylperoxyl radical. The reaction pathway energy profiles and the structure of transition state show that a common feature is the out-of-plane structure of the transition state: in contrast to the energetic minima of a hydrogen-bonded intermediate, the hydrogen bond in transition structures is considerably twisted out of the aromatic ring. From the values of enthalpy (△H) and activation energy (Ea)obtained, it is found that the rate of the reaction of peroxyl radical with phenolic antioxidant is higher than that of alkyl radical with antioxidant. Spin density distributions show that the electron transmission is between methyl (methylperoxyl) radical and phenol.