The self-discharge process of vanadium flow battery (VFB) assembled with Nafion 115 is investigated in very detail for the first time. The self-discharge phenomenon of VFB is closely related to the diffusion coefficients of the vanadium ions, which are found to be in the order of V2+ >VO2+ >VO2 + >V3+. Five regions on the change of open circuit voltage (OCV) are clearly found during the self-discharge process. The regions include three platforms and two obvious decreasing regions. VO2 + disappears in the second region, while the V2+ disappears in the fourth one. In the first three regions, the self-discharge reactions at the positive and negative side are different, owing to the crossover of vanadium ions. In the last two regions, the changes of vanadium ions are derived from the diffusion of V3+ and VO2+ at positive and negative electrolyte. The self-discharge process at different flow rates or different state of charge (SOC) is also investigated, indicating that the self-discharge time shortens with increasing of flow rate between 40 and 80 mL/min or decreasing of the initial SOC. This paper will provide very valuable information for the relaxation or elimination of self-discharge phenomenon of VFB, which is one of the most troublesome issues in VFB application.