Electrical conductivities of various mullite/zirconia composites, as well as monolithic mullite and zirconia, were measured using AC impedance spectroscopy from 100 Hz to 10 MHz at temperatures ranging from 150 to 1300 °C. The impedance spectra of monolithic zirconia and mullite/zirconia composites showed two semicircles because of the contributions from grains and grain boundaries, while those of monolithic mullite had one semicircle due to the predominant contribution from grains. This indicates that the conductivities of the mullite/zirconia composites increased with zirconia content. The activation energies of electrical conduction in mullite and zirconia were about 65 and 79 kJ/mol, respectively, and those of mullite/zirconia composites were between 65 and 79 kJ/mol. While the conductivities of various composites at 1 MHz were fitted by Lichtenecker’s rule, the general mixing equation could be applied to the conductivities measured at 1 kHz.