Water is needed to providing proper hydration of membrane and its ionic conductivity in polymer electrolyte membrane fuel cells, but excess water accumulation known as flooding phenomenon decreases the passing way of reactants in the GDL and reaction sites on catalyst layer and increases mass transport loss and leads to performance loss of polymer electrolyte membrane fuel cells. In the present work, the two-phase flow in the cathode channel of transparent polymer electrolyte membrane fuel cell with single serpentine flow field is visualized by direct optical imaging in unsteady and time averaged states. Then the water coverage length ratio and the average of water coverage length ratio are derived as a scale of water content of the cathode channel in the unsteady and time averaged states. In the unsteady and time averaged states, by increase the stoichiometry, decrease the relative humidity and inlet gases temperature in anode and cathode sides, the accumulated liquid water in the channels reduces. The effect of anode stoichiometry on the amount of water in the cathode channel in the unsteady and time averaged states is more than the cathode stoichiometry.