Optimization of chemical composition and topography is vital to obtain high-performance proton exchange membranes (PEMs). In this work, cellulose nanofibers were incorporated with phosphoric acid and embedded into the sulfonated poly (ether sulfone) (SPES) matrix to develop PEMs with improved proton conductivity by building proton transfer channels and providing additional proton transport sites. The morphology and chemical structure of the nanofibers doped with different phosphoric acid concentrations and the performance of the composite PEMs were characterized by multi-technics. The results showed that the thermal stability, water swelling ratio (SR) and proton conductivity of the composite membrane were improved in the phosphoric acid-doped cellulose nanofibers. The maximum conductivity (0.154 S/cm, 80 degrees C, 100 RH) was reached for composite membranes with cellulose nanofibers doped with 0.25 mol/L phosphoric acid. Hence, composite membranes containing proton-conducting cellulose nanofibers could be used to develop novel PEMs for fuel cells.