Abstract The stack of a vanadium redox flow battery (VRFB), which is a promising energy storage system (ESS), is composed of flow frames (FFs), carbon felt electrodes, bipolar plates (BPs) and membranes. The components of VRFB are assembled and compacted using flat gaskets or O-rings to seal the highly concentrated vanadium sulfuric electrolyte. The reliable sealing of the stack is a crucial issue because the highly concentrated sulfuric acid will damage the system when it leaks. In this work, a gasket-less FF composed of fluoroelastomer/glass fiber composite was developed to prevent leakage without using gaskets or O-rings. The fluoroelastomer layer, which has high chemical stability under an acidic environment, was formed at the surface of the composite, which worked as a sealant of the FF owing to its resilience. Glass fiber reinforcement into the matrix was employed to provide mechanical properties for use as a flow frame structure under the compaction pressure. The surface treatment and fabrication methods for the fluoroelastomer/glass fiber composite were developed. Based on the measured mechanical properties and sealability test of the composite with respect to the curing conditions, the zero-leakage compaction pressure for the gasket-less FF was investigated.