Abstract: The perovskite-type oxide solid solution Ba_0.98Ce_0.8Tm_0.2O_3-α was prepared by high tem-perature solid-state reaction and its single phase character was confirmed by X-ray diffrac-tion. The conduction property of the sample was investigated by alternating current impedance spectroscopy and gas concentration cell methods under different gases atmo-spheres in the temperature range of 500-900 oC. The performance of the hydrogen-air fuel cell using the sample as solid electrolyte was measured. In wet hydrogen, the sample is a pure protonic conductor with the protonic transport number of 1 in the range of 500-600 oC, a mixed conductor of proton and electron with the protonic transport number of 0.945-0.933 above 600 oC. In wet air, the sample is a mixed conductor of proton, oxide ion, and elec-tronic hole. The protonic transport numbers are 0.010-0.021, and the oxide ionic transport numbers are 0.471-0.382. In hydrogen-air fuel cell, the sample is a mixed conductor of proton, oxide ion and electron, the ionic transport numbers are 0.942-0.885. The fuel cell using Ba_0.98Ce_0.8Tm_0.2O_3-α as solid electrolyte can work stably. At 900 oC, the maximum power output density is 110.2 mW/cm², which is higher than that of our previous cell using Ba_0.98Ce_0.8Tm_0.2O_3-α (x≤1, RE=Y, Eu, Ho) as solid electrolyte.