The effects of addition of 3YSZ (3 mol% yttria stabilized zirconia) on electrical and mechanical properties of 8YSZ (8 mol% yttria stabilized zirconia) has been investigated. Three different composites of 3YSZ/8YSZ with compositions of 10 wt.%, 25 wt.% and 35 wt.% of 3YSZ were prepared by conventional solid oxide mixing followed by sintering. Microstructures were characterized by XRD and SEM. The electrical properties were studied by impedance spectroscopy as a function of temperature. Biaxial flexural strength and fracture toughness of composite samples were measured by ring on ring and Vickers microhardness indentation methods respectively. Results of electrical conductivity measurement proved that by increasing the 3YSZ content of the composite samples, the electrical conductivity at high temperatures (T>550 degrees C) decreased while at low temperatures (T<550 degrees C) the conductivity increased. In addition, composite electrolytes had lower activation energy of conductivity at both low and high temperatures. It was shown that both toughness and fracture strength of composite samples improved by the addition of 3YSZ particles. (C) 2008 Elsevier B.V. All rights reserved.