In this study, 8 mol % yttria stabilized zirconia (YSZ) + La0.8Sr0.2MnO3 (LSM) composite electrodes with addition of various amounts of YSZ were prepared on YSZ plates by a screen-printing method. The electrodes were then examined by scanning electron microscopy (SEM) and studied by ac impedance, cyclic voltammetry, and potential step as well as a polarization technique. For the oxygen reduction reaction on the pure LSM electrode, the dissociative adsorption of oxygen on the LSM surface and the transfer of oxygen ions from the triple-phase boundary (TPB) to the YSZ electrolyte lattice were found to be two comparable rate-determining steps. The electrochemical resistance of the former step was proportional to the -0.5 power of p(O2), with a high activation energy of similar to 2.0 eV. The electrochemical resistance of the latter step was found to be independent of p(O2), with a low activation energy of similar to 1.0 eV. With addition of YSZ to the LSM electrode, the electrochemical activity was improved substantially with much lower electrochemical resistances for both steps. Furthermore, the dissociative adsorption of oxygen became less rate determining on the electrode with more YSZ addition. It was found that the transfer of oxygen ion was the only rate-determining step on the 40% YSZ + LSM electrode. The improvement in the electrochemical performance with addition of YSZ was found mainly due to the spatial enlargement of the TPB area, which increased the electrochemically active sites for the oxygen adsorption and charge-transfer reaction. The intimate sintering of YSZ particles to the YSZ substrate and the subsequent formation of the spatial TPB area were directly found by SEM examination and further confirmed by the increasing cathodic currents from the TPB area, which were separated from the total currents using a potential-step technique.