Among the most critical issues associated with redox flow batteries (RFBs), one is the undesirable crossover of redox-active species through the membrane, which is typically polymer-based. To address this issue, we report an inorganic mesoporous silica film based nanochannel membrane with highly ordered, vertically aligned, and subnanosized channels. Unlike conventional polymer-based membranes that have ion transport channels with fixed sizes between 2 and 4 nm in diameter, the ion transport channels of the membrane are controllable in size and uniform in size distribution, enabling the membrane to separate the ions that are larger than the critical value. Experimental results prove the expected effect that the pristine nanochannel membrane (with the channel size of ∼0.5 nm) grown on the ITO glass can completely separate protons and vanadium ions. After ITO etching, the free-standing membrane maintains extremely low vanadium-ion permeability that is four orders of magnitude lower than that of Nafion membranes, suggesting that it is a promising candidate for redox flow batteries. Moreover, the membrane fabrication is convenient, inexpensive, and does not rely on sophisticated facilities or conditions, providing potential in scalable productions.