In this work, a numerical model is developed to investigate the ion crossover in the vanadium redox flow battery (VRFB). The Donnan effect is considered by introducing an interfacial sub-model at the membrane/electrode interfaces based on the electrochemical potential equilibrium. Using this model, the diffusion and electric field effects on vanadium ions crossover through both the cation- and anion-exchange membranes are quantitively analyzed and the corresponding ion transport mechanisms are explored. The results show that the electric field influences the ions distributions in the membrane thus the transport of ions across the membranes. The properties of ion and membrane are important parameters to determine the electric field induced ion flux. The different dominant transfer ion results in the migration flux and electro-osmotic flux the same directions in the cation-exchange membrane but opposite directions in the anion-exchange membrane, thus a weaker electric field effect for the latter. The net vanadium flux presents the asymmetric characteristic with SOC and current density for both cation-and anion-exchange membranes. Finally, the present work predicts the accumulation of vanadium ions in the positive half-cell for the cation-exchange membrane but an opposite trend for the anion-exchange membrane with cycling. (c) 2019 The Electrochemical Society.