This paper aims to numerically assess the effects of electrolyte properties and electrode morphology on the capacitance of electric double layer capacitors (EDLCs) made of mesoporous electrodes consisting of ordered cylindrical pores in non-aqueous electrolytes. Simulations solved a three-dimensional modified Poisson-Boltzmann model. They accounted for the finite size of ions and field-dependent electrolyte permittivity while the pores were perpendicular to the current collector. The effects of pore radius, porosity, effective ion diameter, and electrolyte field-dependent permittivity on the diffuse layer gravimetric capacitance were investigated systematically in order to determine key parameters affecting EDLCs' performance. The simulations showed that reducing the ion effective diameter and the pore radius resulted in the strongest increase in diffuse layer gravimetric capacitance up to a critical radius below which the capacitance reaches a plateau. Increasing the electrode porosity also increased the diffuse layer gravimetric capacitance. Accounting for more realistic field-dependent permittivity was found to significantly reduce the predicted diffuse layer gravimetric capacitance. Finally, accounting for the contribution of the Stern layer to the total capacitance was essential in predicting experimental data for a wide range of porous activated carbon electrodes and non-aqueous electrolytes. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3622342] All rights reserved.