The mechanism of oxygen reduction at MnO2-catalyzed air cathode was investigated by measurements of polarization curves in wide OH− concentrations, oxygen pressures and using different crystalline MnO2 catalysts. It is found that the oxygen reduction at MnO2 catalyst is accompanied with the reduction of MnO2 and the catalytic activities of MnO2 is closely related to the electrochemical activities of MnO2. Based on the experimental determination of the reaction orders of OH−, O2 and the intermediate Mn3+ ion, we propose a chemical oxidation mechanism for the catalytic oxygen reduction, in which oxygen reduction proceeds through chemical oxidation of the discharge product of MnO2 rather than through a direct two-electron reduction. The rate expression derived from this mechanism can very well explain the observed polarization properties and the concentration dependence of oxygen reduction.