Ceria-samaria (CeO2-Sm2O3) is one of the most interesting of fluorite oxides since its oxide ionic conductivity is higher than that of yttria-stabilized zirconia and other CeO2-based oxides. In this study, the effective index is proposed to maximize the oxide ionic conductivity in CeO2-Sm2O3 based oxides. Considering the crystallographic character of fluorite structure in CeO2-Sm2O3 system, the oxygen vacancy level was combined with the ionic radius mismatch between host and dopant rations as criteria for fast ionic conduction. SmxCe1-xO2-delta (0.2 < x < 0.3), (Sm0.5C0.5)(x)Ce1-xO2-delta (1.175 < x < 0.3), and (Sm0.936Cs0.06Li0.004)(x)Ce1-xO2-delta (0.2 < x < 0.275) were prepared and characterized as examples with low, intermediate, and high index, respectively. The oxide ionic conductivity increased with an increase of the effective index, confirming the validity of the index. Furthermore, the electrochemical properties of Sm0.25Ce0.75O1.88, (Sm0.5Ca0.5)(0.225)Ce0.775O1.84, and (Sm0.936Cs0.06Li0.004)(0.25)Ce0.75O1.86 were investigated in the temperature range from 700 to 1000 degrees C as representative examples. The oxide ionic conductivity in reducing atmosphere improved with increasing index. It was found that (Sm0.936Cs0.06Li0.004)(0.25)Ce0.75O1.86 With the highest index value in the CeO2-Sm2O3 system, exhibited high oxide ionic conductivity and good performance in planar solid-oxide fuel cells.