To simulate impedance spectra of yttria-stabilised zirconia/alumina composites a digital-image based model has been developed. The computer simulation generates the polycrystalline microstructure of the material by exploiting the Voronoi tessellation technique. Then a 3-D network of cubes is superimposed onto the simulated polycrystalline microstructure and the edges of each cube substituted with discrete RC parallel electrical circuits. The composite is, in such a way, converted into a three-dimensional electrical network whose impedance has been calculated at different frequency values. The numerical solution has been performed via an iterative method based on a matrix non-linear recursion relation. Results of the simulation are presented and compared with experimental data.