The major objectives of this project are to identify the fast and effective membrane –electrode assembly (MEA) activation procedure and to understand the MEA activation mechanism. In order to do that, the effect of three on-line activation procedures (constant current (0.25 A cm -2 ), constant voltage (0.6 V), and USFCC protocol) on the MEA performance were investigated. Electrochemical tests results indicated that constant potential activation procedure at 0.6 V for 6 h is an effective and fast method compared to constant current and USFCC activation protocols. For all studied procedures, the values of ohmic, charge and mass resistances were decreased during the activation process. The more detailed performance investigation of the MEA activated by USFCC activation procedure indicated that the order of oxygen reduction reaction and normalized current density at 0.9 V are 0.8 and 0.4 mA cm -2 , respectively. The absolute values of these parameters are consistent with the ones reported in literature related to other MEAs. The effect of mentioned MEA activation procedures on the final MEA performance and EIS response was studied under different operation conditions. MEA conditioning procedure had an irreversible effect on the MEA interfacial properties which could not be corrected even in wide range of operational conditions. MEA activation by constant low current density was not an effective procedure compared to constant voltage (0.6 V) and USFCC activation protocol to reach to the maximum performance of MEA even for a long time of activating, and under different operation conditions. Maximum power density of MEA activated by constant current 0.25 A cm -2 was 20% lower than that of other MEAs at cell temperature of 75 o C, pressure of 34 kPa and cathode relative humidity of 30%. The effect of operation condition of MEA activation procedure on the MEA performance was investigated. Results indicated that MEA activation at the high temperature and pressure reduces the time of activation at least 30%. MEA activation did not fully occur at the high temperature and pressure operation condition and the activation procedure could not bring the MEA to its highest and stabilized performance at the steady-state condition. In the last part of this project, ultrasonic bath cleaning technique was successfully applied for the pre-treatment of the gas diffusion electrode before membrane-electrode assembly. The results indicated that ultrasonic bath pre-treatment significantly improves MEA performance in all current density regions at different operation conditions. This pre-treatment technique also reduced the time of the MEA on-line activation procedure.