Considering the existing approaches in the study of fuel cells durability, in this thesis the durability of fuel cells was examined in three sections. In the first part, the effect of cathode catalyst layer void volume on the long-term performance of the polymer electrolyte membrane fuel cell has been investigated. The results showed that the cathode catalyst layer void volume plays an important role in the long-term performance of the fuel cell. The optimization of this parameter, which determines the available three phase regions and routes of mass transfer in the membrane electrode assembly (MEA), improves the short-term and long-term performance of the fuel cell. The results showed that for the MEA with cathode catalyst layer porosity of 20.8%, an optimal structure and a proper balance between catalyst layer void volume and Nafion content is obtained. So that it’s maximum output power density has changed only 0.8% during 200 cycles. In the second section, the effect of two conventional MEA activation methods (constant current and constant voltage methods) on the long-term performance of fuel cell is investigated. According to the obtained results, the activated MEA by constant current method showed an average voltage decay of 11.33 ?V/cycle at 1 A/cm 2 , compared to 4.4 ?V/cycle for the activated MEA by constant voltage procedure. It was due to the more reduction of electrochemical surface area for the activated MEA by constant current method (32% vs. 19%). Also, after 9000 degradation cycles, more severe platinum nanoparticles agglomeration was seen in the cathode catalyst layer of activated MEA by constant current procedure. This showed that MEA activation by constant current activation method not only need to the longer activation time, but also causes higher catalyst layer degradation. In last section of this thesis, the effect of operating temperature and pressure on the long-term performance of dead-ended anode fuel cell is investigated. According to the results, it was demonstrated that operation of dead-ended anode fuel cell at high temperature and pressure reduce the durability of membrane electrode assembly. In such a way that after 9000 degradation cycles, the maximum power density under H 2 /O 2 gas feed mode for the aged MEA at high temperature and pressure was dropped by 38.8%. While the maximum power density drop was 27.1% for the aged MEA at low temperature and pressure. This suggested the more severe destruction of catalyst layer at higher temperature and pressure. In addition, the transmission electron microscopy and scanning electron microscopy images showed the further degradation of cathode catalyst layer and more sever Pt agglomeration at higher temperature and pressure.