Oxide-ion conductivity of pyrochlore-type solid solution was measured for the compositions of (Yb_1-xNd_x)₂(Ce_1-0.88xZr_0.88x)₂O₇ (YbNd system), which had various average cation radius ratio, r(A³⁺)/r(B⁴⁺), but constant lattice parameters and oxygen content. The crystal structure of the YbNd system were assigned to rare-earth C-type (x=0), fluorite-type (0.2≦x≦0.8) and pyrochlore-type (x=1.0) phases, depending on the average cation radius ratio of r(Yb_1-xNd_x)/r(Ce_1-0.88xZr_0.88x). The oxide-ion conductivity of the YbNd system decreased with an increase in the ordering of oxygen vacancy. These results were supported by the results of Raman spectra of the YbNd system.