The electrical conducton in the sintered 3Bi2O3.WO3 solid electrolyte was investigated by measuring the conductivity and ionic transport number. The electrical conductivity was about three to ten times higher than that of YSZ at temperatures between 300 and 80$0^{circ}C$. D.C. polarization method confirmed that 3Bi2O3.WO3 was predominantly an ionic conductor. Unlike the instability of high conductive fcc phase in the rare-earth oxide-Bi2O3 or Y2O3-Bi2O3 systems at temperature below $700^{circ}C$, fcc phase in the 3Bi2O3.WO3 exhibited no transformation even after annealing over 900 hrs at 600 and $650^{circ}C$. Two samples which had different grain sizes showed almost the same conductivity. This result suggests that the electrical properties of grain and grain boundry were very similar.