In this paper we study the performance of a PEM stack fuel cell after 900 hours of operation. We tried to identify any voltage degradation and its causes. Microscopic cracks on some parts of MEA mainly due to uniformity of pressure, stress concentration and elongation of membrane in the edge of GDL has been identified as the most important cause of this gradual degradation. High contact resistance between layers of stack, including the membrane, is another cause of PEM stack degradation. All problems are solved and tested by Electrochemical Impedance Spectrometry and the best amount of thickness of the membrane is determined. The result of EIS test with optimized sealing washer shows that the stack operation was optimized with the decrease of contact resistance in the several layers of stack in the active area of membrane.