A polymer composition, a cross-linked polymer formed therefrom, an electrolyte membrane and electrode for a fuel cell including the same, and a fuel cell employing the electrode and the electrolyte membrane. The cross-linked polymer includes an oxazine-based monomer capable of retaining phosphoric acid and has excellent mechanical strength. When the cross-linked polymer is used, an electrode and electrolyte membrane for a fuel cell with enhanced capability of retaining phosphoric acid at a wide range of temperature may be manufactured, leading to long-term durability, and enhanced proton conductivity.