The present study was carried out to find out the best combination of gas diffusion layers (GDLs) for a polymer electrolyte fuel cell (PEFC) operated under very low-humidity conditions at the cathode and high-humidity conditions at the anode using a test apparatus in which the clamp pressure on the cell could be accurately controlled. For cathode GDLs, it is important to maintain humidity and prevent dehydration of the MEA. A microporous layer (MPL) coating on the low-porosity carbon paper substrate is effective for enhancing PEFC performance. For anode GDLs, however, it is important to increase water transport from the humidified hydrogen gas to the MEA. An anode GDL with the MPL reduces PEFC performance. Hydrophobic treatment by PTFE (polytetrafluoroethylene) loading in the anode GDL is effective for performance enhancement. However, when the PTFE content becomes too high, the water transport properties are reduced, thereby lowering PEFC performance.