Complex alternating current impedance and steady-state polarization measurements have been conducted on dense and thin LaMnO3 and La0.85Sr0.15MnO3 film electrodes and porous-sintered LaMnO3 and La0.85Sr0.15MnO3 electrodes in air at elevated temperatures between 873 and 1273 K, in order to study the reaction mechanism of oxygen reduction at the La1-xSrxMnO3 electrode of a solid oxide fuel cell. By fitting impedance spectra to an appropriate equivalent circuit, the chemical diffusion coefficient of oxygen and interfacial reaction resistance of the LaMnO3 and La0.85Sr0.15MnO3 film electrodes were determined. The chemical diffusion coefficient was scarcely affected by Sr doping, while the interfacial reaction resistance considerably decreased by Sr doping. Steady-state polarization behavior of the porous-sintered La1-xSrxMnO3 was dramatically improved by doping Sr, while those of the dense La1-xSrxMnO3 film were almost unchanged by Sr doping. These results suggested that the electrochemical reduction of oxygen at the porous La1-xSrxMnO3 electrode takes place around the triple phase boundary (TPB), and the reaction rate is controlled by the surface reactions close to the triple phase boundary region.