This paper studied the breakthrough pressure for liquid water to penetrate the gas diffusion layer(GDL) of a proton exchange membrane fuel cell(PEMFC).An ex-situ testing was conducted on a transparent test cell to visualize the water droplet formation and detachment on the surface of different types of GDLs through a CCD camera.The breakthrough pressure,at which the liquid water penetrates the GDL and starts to form a droplet,was measured.The breakthrough pressure was found to be different for the GDLs with different porosities and thicknesses.The equilibrium pressure,which is defined as the minimum pressure required maintaining a constant flow through the GDL,was also recorded.The equilibrium pressure was found to be much lower than the breakthrough pressure for the same type of GDL.A pore network model was modified to further study the relationship between the breakthrough pressure and the GDL properties and thicknesses.The breakthrough pressure increases for the thick GDL with smaller micro-pore size. This paper studied the breakthrough pressure for liquid water to penetrate the gas diffusion layer (GDL) of a proton exchange membrane fuel cell (PEMFC). Ann ex-situ testing was conducted on a transparent test cell to visualize the water droplet formation and detachment on the surface of different types of GDLs through a CCD camera. breakthrough pressure, at which the liquid water penetrates the GDL and starts to form a droplet, was measured. breakthrough pressure was found to be different for the GDLs with different porosities and thicknesses The equilibrium pressure, which is defined as the minimum pressure required maintains a constant flow through the GDL, was also recorded. The equilibrium pressure was found to be much lower than the breakthrough pressure for the same type of GDL. A pore network model was modified to further study the relationship between the breakthrough pressure and the GDL properties and thicknesses. breakthrough breakthrough increase for the thick GDL with small er micro-pore size.