Some acceptor-doped perovskite type oxides exhibit proton conduction. These materials are of special interest because of their potential application as a solid electrolyte used in H-2/O-2 fuel cells. In order to increase the power density as well as to reduce the operation temperature of the fuel cells, development of the electrolyte materials with high proton conductivity is required. In the present study, we measure dc conductivity on the compositionally graded proton conducting oxides, and reveal the existence of the compositionally gradient effect in these oxides. That is the dc conductivity for the downhill configuration, with respect to the chemical potential for the hydration, is about three times larger than that for the uphill configuration. Moreover, the internal resistance of the fuel cells based on the compositionally graded electrolyte of the downhill configuration is reduced 25similar to30% compared with that of the uphill configuration.