Acid doped polybenzimidazole membrane has been prepared for proton exchange membrane fuel cell. The present work aims at improving the proton conductivity without losing mechanical properties at high doping level by modifying poly (2,5-benzimidazole) (ABPBI). The nitration of ABPBI has been carried out with acid mixture of HNO3 and H2SO4 at 30oC. This nitro compound has been reduced successfully by stannous chloride/dimethylacetamide in presence of hydrochloric acid. Fourier transform infrared (FT-IR) spectra confirm the presence of primary amine groups in the ABPBI structure. The primary amine functionalized ABPBI has been cross-linked by hexamethylene diisocyanate and casted onto glass Petridis using dimethyl sulfoxide as solvent. Membrane doped with phosphoric acid shows comparatively higher proton conductivity, but lower tensile strength.