Water plugging in cathode flow field of polymer electrolyte fuel cells (PEFCs) is one of the critical issues to be solved for high power density and high durability. To alleviate performance degradation due to water plugging, it is necessary to control liquid water transport inside gas channel and promote water removal. In this study, the in-situ visualization of liquid water behavior in the serpentine flow field of a specialized transparent PEFC was performed using a high-speed camera. Furthermore, the cell voltage and the differential pressure between the cathode inlet and outlet were simultaneously measured, and the effects of channel wettability and flow direction on the water transport and cell performance were investigated under operating conditions. It was found that the horizontal arrangement of the serpentine channel enhances liquid water removal from the cell without being disturbed by gravity. Especially, highly hydrophilic coating of the horizontally oriented serpentine channel effectively suppresses water plugging and reduces the differential pressure inside the cathode flow field. (C) 2014 Elsevier B.V. All rights reserved.