The structure and electrical properties of an A-site-deficient perovskite compound found in the La2/3TiO3-La1/3NbO3 System were investigated. The composition of the perovskite compound seemed to be very close to La-0.633(Ti0.90Nb0.10)O-3. X-ray diffraction analysis revealed a superstructure with a doubled c-axis parameter, resulting from an ordered arrangement of the A-site cation vacancies. Impedance measurements on the compound showed that La-0.633(Ti0.90Nb0.10)O-3 had high ionic conductivity at relatively low temperature (<770 K) and increased electronic conduction at high temperature (>770 K). The bulk ionic conductivity was comparable with that of La-0.683(Ti0.95Al0.05)O-3, which has the highest ionic conductivity among the La-(Ti,Al)-O perovskite compounds.