A multiple physical finite elements to analyze software (COMSOL Multiphysics) is employed to investigate serpentine flow channels of a small proton exchange membrane fuel cell with power 0.5W. Distributions of the oxygen flow, the liquid water, and the electric current density in three different designed flow channels are simulated and compared. Results show that increasing the channel width is conducive to increase the oxygen flow and reduce the production of liquid water. It thus enhances the electric current density. However, it will reduce the oxygen flow, produce more liquid water, and depress the current density if the channel width is decreased.