(LNF) is one of the promising cathodes for solid oxide fuel cells, but reacts with a zirconia-based electrolyte. To prevent this undesirable reaction, a ceria phase has been introduced in between the LNF cathode and electrolyte. On the other hand, the ceria phase itself could react with lanthanum-based perovskite oxides. We examined the reactivity of LNF and (samaria doped ceria (SDC)) in this study. The mixtures of LNF and SDC were sintered at temperatures between and and the resultants were analyzed by X-ray diffraction together with the Rietveld analysis. We also measured the activity of electrochemical cells with a LNF-SDC composite layer in between the LNF cathode and zirconia-based electrolyte. The lattice parameters of each phase are clarified and a possible reaction scheme is proposed. The cell activity was high, but was influenced by the sintering temperature of the composite. Both chemical stability and physical property of the cathode can affect the cell activity.