The reactivity of radical-molecular addition reaction was studied in terms of density-functional theory (DFT) and Hartree-Fock theory (HFT). It was found that the DFT method can give more accurate estimated result than the HFT method. The reason might be that the energy of the highest occupied molecular orbital (HOMO) in DFT is not the ionization energy,but rather represents the change in the total energy with respect to the change in occupation number of electrons in the HOMO,consistent with the concept of the transition state. The energy of HOMO in HFT is the ioniza-tion energy,which may represent more properly the prop-erty of the products and reactants themselves.