In this paper, the dielectric relaxation of SiO2 nano-particle suspensions is studied by electrical impedance spectroscopy method. The case of suspension composed of particles with thick double layer is concerned. The objective of this study is to find out the relationship between particle properties (size and concentration) and electrical impedance spectroscopy for developing a particle characterization method based on electrical impedance spectroscopy/tomography. The influences of particle size on electrical impedance spectroscopy (EIS) and relaxation frequency are investigated and analyzed. The experimental results indicate that impedance phase angle and a relaxation frequency are functions of particle size. a relaxation frequency is proportionally changing with (a+kappa(-1))(-n) (a is particle radius, kappa(-1) is double layer thickness, n equals 0344 for 4.76% suspension and 0308 for 238% suspension). The exponent term, n, is smaller than the one in dilute suspensions (n = 2), which is possibly due to decreasing of diffusion distance of ions around the particles in concentrated suspensions with thick double layer. The impedance parameters, including conductivity increment, Delta K', and alpha relaxation frequency are influenced by particle volume fraction. The conductivity increment, Delta K' becomes less negative with increasing particle volume fraction due to the positive contribution of double layer charge on the conductivity increment. The a relaxation frequency increases with increasing particle volume fraction and the small particles show a more significant increase than large particles. The experimental result on the differential electrical impedance tomographic images between the silica suspension and water (with same conductivity value) shows that small differences on the values of impedance imaginary part and phase can be observed at the upper right corner in the EIT images, which represent a small differentiation on the dielectric property caused by the electrical polarization of double layer on the particle surface. (C) 2015 Elsevier B.V. All rights reserved.