This study discusses the relationship between performance and carbon fiber diameter (400 nm–1 μm) in fuel cells employing carbon fiber paper produced from PAN fiber felt, and also examines the effect of carbon fiber paper thickness, air permeability, porosity, and surface resistivity on performance. The researchers fabricate gas diffusion layers (GDLs) with a small carbon fiber diameter from PAN fiber employing the two processes of stabilization and carbonization, and investigate the relationship between fiber diameter and air permeability in the gas diffusion layer material. Carbon fiber paper made in this study is left as is or impregnated with 10 wt% phenolic resin or FEP. When the tested area is 25 cm2, the test temperature 40 °C, and the carbon fiber paper impregnated with 10 wt% phenolic resin, the paper has a fiber diameter of 1 μm and an air permeability is 29 cm3 cm−2 s−1, and a test fuel cell yields 997 mA cm−2 at a load of 0.5 V. Carbon fiber paper impregnated with 10 wt% FEP has a smaller carbon fiber diameter of 400 nm and an air permeability of only 1 cm3 cm−2 s−1; a test fuel cell made with this material yields 683 mA cm−2 at a load of 0.5 V.