AMPS polymer membranes containing various amounts of water were fabricated to simulate the material degradation of in-service membranes in PEFC. The life time of PEFC was measured along with measurements of tensile properties and swelling of the membranes. It was concluded that (1) the failure of PEFC is caused by fracture of the AMPS membrane; (2) the tensile strength of the AMPS membrane decreases with increasing water content due to a softening of the membrane; and (3) a high concentration of the cross-linker molecules is very effective for improving the mechanical strength of the membrane and the resistive force against the swelling, which results in the longer durability of PEFC.