We report herein that N-substituted poly(2,5-benzimidazole) (ABPBI) membranes can be applied vanadium redox flow batteries (VRFBs) with outstanding performance. ABPBI was prepared by the polycondensation of 3,4-diaminobenzoic acid (DABA) in polyphosphoric acid, substituted ABPBI was synthesized with different substitution ratios. As the substitution ratio increases, crystallinity of substituted ABPBI membrane was gradually changed from crystal to amorphous structure. Also, ABPBI membrane with amorphous structure showed higher water and sulfuric acid absorption properties, and exhibited lower area resistance than ABPBI membranes with crystal structure. The VRFB assembled with ABPBI-50 showed outstanding cell performance and long term stability. Also, outstanding physicochemical stability of ABPBI-50 was confirmed by NMR analysis. All experimental results suggest the ABPBI membrane with amorphous structure is a promising material for VRFB applications.