Ab initio calculations on O₃, FeO₂, Fe(Ⅰ)O₂, Fe(Ⅱ)O₂, CoO₂ and Co(Ⅱ)O₂ were carried out by the spin unrestricted Hartree-Fock method. The results show that the Mulliken population of Fe^(Ⅱ), central Oc and terminal Or are 24.18, 8.19 and 7.64 respectively for π-system in oxyhemoglobin. This is contrary to the suggestion by Weiss that the Fe-O₂ bond is similar to Fe³⁺O₂^-. The result appears to fit the experimental data very well and provides good mechanistic evidence for explaining the oxygenation-deoxygenation of oxyhemoglobin in the life of living organisms.