Abstract The Psf–PPSS–TPA composite cation exchange membrane consist of Psf(polysulfone)–PPSS (polyphenylenesulfidesulfone) block copolymer with TPA (tungstophosphoric acid) is prepared to apply for a separator in the all-vanadium redox flow battery. The membrane properties such as membrane resistance and ion exchange capacity, and thermal stability are investigated. The prepared Psf–PPSS–TPA composite cation exchange membrane showed higher thermal stability than Nafion117. The lowest membrane resistance of the prepared Psf–PPSS–TPA composite cation exchange membrane measured in 1 M (mol/dm 3 ) H 2 SO 4 aqueous solution was 0.94 Ω cm 2 at 0.5 g of TPA solution. The performance properties of the all-vanadium redox flow battery (V-RFB) using the prepared cation exchange membrane are measured. The electromotive force, open circuit voltage at state of charge (SOC) of 100%, was 1.4 V. This value meets a theoretical electromotive force value of the V-RFB. The measuring cell resistance in charge and discharge at SOC 100% were 0.26 Ω and 0.31 Ω, respectively. The results of the present study suggest that the prepared Psf–PPSS–TPA composite cation exchange membrane is well suited for use in V-RFB as a separator.