The relationship between electrical conductivity and crystal structure was investigated for Ln(2)Zr(2)O(7) (Ln = La, Nd, Sm, Eu, Gd, Y, or Yb) and (Ln(1-x)Ln(x)')(2)Zr2O7 (Ln=Gd, Sm, or Nd; Ln'=Y, Yb, or Gd) systems. The crystal structure of both systems changed from fluorite (F)-type to pyrochlore (P)-type structure when the ionic radius ratios; r(Ln(3+))/(Zr4+) or r(Ln(av)(3+))/r(Zr4+), were larger 31 than 1.26, where r(Ln(av)(3+)) is estimated from the ionic radius of the component ions and the composition using the following equation: r(Ln(av)(3+))=(1 -x)r(Ln(3+))+xr(Ln(3+)). The lattice parameter increased linearly with increasing ionic radius ratios. The electrical conductivity at 800 degreesC in air for Ln(2)Zr(2)O(7) systems showed the sharp maximum at the vicinity of the phase boundary between fluorite- and pyrochlore-type phases. The electric al conductivity of (Ln(1-x),Ln'(x))(2)Zr2O7 system also showed the maximum at the phase boundary for some combinations of Ln(3+) and L8+. The pyrochlore-type Eu2Zr2O7, which is located at the nearest position to the phase boundary, showed the highest conductivity of 8.3 x 10(-3) S cm(-1) at 800 degreesC. On the other hand, the activation energy for the conduction remarkably decreased with the increasing ionic, radius ratios in the fluorite-type phase range and showed the minimum at the given compositions, at which the maximum electrical conductivities were observed and then increased.