The electrical properties correlated with microstructures of dense and porous three dimensional (3D) nanocrystalline Ce0.9Gd0.1O1.95 (n-CGO) electrolytes prepared by the different sintering processes were investigated. The CGO pellets of dense-nano (DN-CGO), and porous-nano (PN-CGO) with grain size of 30 similar to 70 nm and their equivalent density pellets of dense coarse (DC-CGO) and porous coarse (PC-CGO) with larger grain size were prepared. The preparation of nanocrystalline CGO electrolytes with different grain sizes and densities was dependent on the different sintering processes, by controlled heating profiles, such as one step sintering and two steps sintering techniques. The electrical conductivity of CGO pellets was characterized using ac impedance spectroscopy (IS) as a function of temperature, oxygen partial pressure and grain size. The comparable electrical conductivity behavior for DN-CGO and DC-CGO may attribute to distant grain size level of DN-CGO from critical nano-grain size. On the other hand, a decreased electrical conductivity was observed in PN-CGO with grain size of 30 nm compared to its equivalent PC-CGO, including the porous conductivity correction, which confirms the existence of space charge region affecting the defect characteristics of nanocrystalline CGO. The electrical transport properties and microstructure parameters of the 3D nanostructured CGO electrolyte were discussed using the space charge model. (C) 2008 Elsevier B.V. All rights reserved.