Polybenzoxazine (PBz)-modified polybenzimidazole (PBI) nanofibers have been prepared by an electrospinning process. The nanofibers have been crosslinked through the ring-opening addition reaction of the benzoxazine groups of PBz. The crosslinked PBI nanofibers show an enhancement in mechanical strength and an increase in resistance to solvents, so are of use as a reinforcement for the preparation of PBI-based composite membranes. Modification of the PBI-based composite membranes with the crosslinked PBI nanofibers significantly improves their mechanical properties, acid uptakes, and dimensional stability upon acid doping. The composite membrane shows a proton conductivity of 0.17 S cm(-1) at 160 degrees C under anhydrous conditions, which is about 2-fold higher compared to the proton conductivity of the neat PBI membrane. The single cell employing the composite PBI membrane exhibits a maximum power density of 670 mW cm(-2), which is about a 34% increase compared to the cell employing the neat PBI membrane. A convenient and effective approach to prepare high performance proton-exchange membranes for high temperature fuel cells has been demonstrated.