A crosslinked object of a polybenzoxazine-based compound formed of a polymerized resultant of a first monofunctional benzoxazine-based monomer or a second multifunctional benzoxazine-based monomer with a crosslinkable compound, an electrolyte membrane including the crosslinked object, a method of preparing the electrolyte membrane, and a fuel cell employing the electrolyte membrane including the crosslinked object. The crosslinked object has a strong acid trapping capability with respect to the benzoxazine-based compound and high mechanical properties due to the crosslinking. The crosslinked object is very stable chemically because of elimination of solubility in polyphosphoric acid. The electrolyte membrane including the crosslinked object has excellent phosphoric acid supplementing capacity at a high temperature and mechanical and chemical stability. Specifically, even when an impregnated amount of a proton carrier, such as phosphoric acid, is increased to increase proton conductivity, the electrolyte membrane maintains excellent mechanical and chemical stability. Accordingly, the electrolyte membrane can be used in a fuel cell for high temperature and no humidity.