Poly(tetrafluoroethylene) reinforced sulfonated poly(ether ether ketone) (SPEEK/PTFE) composite membranes are prepared for vanadium redox flow battery (VRB) application. Results show that SPEEK/PTFE composite membranes have lower water uptake and swelling ratio than that of SPEEK membranes due to the PTFE reinforcement. As a result, the composite membranes show higher elongation ratio and better mechanical stability than SPEEK membranes. VRB single cell tests are also carried out to further evaluate their performance. The batteries assembled with SPEEK/PTFE membranes exhibit higher columbic efficiency (CE) and energy efficiency (EE) than that of SPEEK membranes. Furthermore, the composite membranes show much better stability than pristine SPEEK, confirming that PTFE can effectively reinforce membranes to improve their chemical and mechanical stability under VRB operating condition.