During cold startup of polymer electrolyte fuel cells (PEFCs), water freezing in the porous cathode is a critical barrier to achieving high power density. In this study, the freezing of water inside the cathode of a PEFC during low temperature operation was directly visualized using an optical diagnostic, and the correlation between the ice formation and the cell voltage characteristic was investigated. The effects of the structure of the gas diffusion layer (GDL) and operating conditions on the water freezing and cell performance were also discussed. It was found that the water generated in the cathode does not freeze immediately after starting the operation at -10 degrees C. When the liquid water within the cathode freezes during the cold startup, the cell voltage drops due to the limitation of oxygen diffusion. Furthermore, the operation time below the freezing point is largely affected by the material properties of GDL, current density and cathode gas composition.