In this study, we suggest an experimental method to accurately understand the unique performance of membrane electrode assembly (MEA) according to the simulated environment (relative humidity and oxygen concentration) by using a small cell and supplying excess gas flow for minimizing environmental gradient during cell operation. Through this, we examined the characteristics of the electrocatalysts and composition of electrodes which maximize the performance of the large-area fuel cell, and also found the composition that minimizes deviations from non-uniformity during large-area operation. Furthermore, we also studied the performance characteristics of the MEAs according to the physicochemical properties of catalysts, and microstructures from catalyst and ionomer. The unit cell experiment confirmed that different performance characteristics of MEAs results from the structure of carbon support of each catalyst. In addition, the performance characteristics of MEAs at different operating voltages varied depending on the location of Pt nanoparticles on carbon support caused by its structural characteristics.