The effect of grain boundaries on electrical conductivity was investigated in sintered (Bi2O3)(0.75)(WO3)(0.25) and (Bi2O3)(0.7)(SrO)(0.3) solid electrolytes. Electrical conductivities of the specimens with different grain sizes were measured using ac impedance analysis. The grain conductivity and the grain boundary conductivity were not separated in the complex impedance plane, and the specimens with large grain size showed almost the same conductivities as the specimens with small grain size, In the Bi2O3 system, it is thought that the electrical properties of the grain boundary are not different from those of the grains regardless of their crystal structures.