Crosslinked polybenzoxazines obtained by crosslinking a monofunctional first benzoxazine monomer and a multifunctional second benzoxazine monomer with a crosslinkable compound, an electrolyte membrane including the same, a method of preparing the electrolyte membrane, a fuel cell including the electrolyte membrane having the crosslinked polybenzoxazines using the method. The crosslinked polybenzoxazines have strong acid trapping capability, improved mechanical properties, and excellent chemical stability as it does not melt in polyphosphoric acid. Even as the amount of impregnated proton carrier and the temperature are increased, mechanical and chemical stability is highly maintained, and thus the electrolyte membrane can be effectively used for fuel cells at a high temperature.