The effect of percolation on the electrochemical performance of lanthanum strontum manganate (LSM)-yttria-stabilized sirconia (YSZ) composite electrodes was investigated. It was identified that the polarization resistance of the LSM-YSZ composite electrodes is closely related to the connectivity of LSM-YSZ particles and depends on their size ratio. The composite electrode showed low electrochemical activity without YSZ connectivity. However, the polarization resistance decreased abruptly when YSZ connectivity occurred. Because the three phase boundary is dependent on the particle size of LSM and YSZ, the polarization resistance of the cathode decreases along with the decrease of the particle size of LSM and YSZ.