New electrolytes of cubic L $a_2$M $o_2$-x Nbx $O_{9-}$$delta$/(x=0, 0.05, 0.1, 0.2) were fabricated by a solid-state reaction method and their sintered densities were approximately 93％ of theoretical density. X-ray diffraction analysis and microstructure observation for the sintered specimens were performed. The at complex impedance were measured at 34$0^{circ}C$ to 93$0^{circ}C$ in air and fitted with a Solatron ZView program. Their impedance spectra showed big difference below and over the phase transition temperature (58$0^{circ}C$). The electrical conductivity of L $a_2$M $o_2$ $O_{9}$ was 1.36${ imes}$10$^{-2}$ S$cm^{-1}$ / at 817$^{circ}C$. Comparing to undoped L $a_2$M $o_2$ $O_{9}$ , Nb-doped specimen showed the increase of electrical conductivity due to the aliovalent doping effect..