5-cell DMFC stack was manufactured with the 50-cm2 MEAs (Membrane Electrode Assembly’s) and operated for 4000 hrs by the current interrupt method. The stack power was approximately 8.4 W at the initial state. The voltage of the stack decreased slightly under constant load and showed the 18% decrease of stack power after the operation of 2,000 hrs. After a shutdown of 3 days, the stack performance was recovered upto 95% of the initial state under the same operating conditions. The stack performance was influenced by the temperature of test room. The performance of the stack decreased about 25 % after the operation of 4000 hrs with different degree of performance degradation of each cell. The stack was disassembled after the operation for 4000 hrs. The color of the electrolyte membrane changed to dark brown. The Pt particle size was measured at the poisons of methanol inlet and methanol outlet for anode catalyst layer and at the positions of air inlet and air outlet for cathode catalyst layers. Pt particle sizes of the cathode catalyst did not change although that of some anode catalysts increased. Ru crossover from anode catalyst layer to cathode catalyst layer was found in the used MEAs from SEM-EDX experiment. Ru crossover was most for the cell which shows the highest performance degradation.