Clogging of the cathode channel by water during operation of polymer electrolyte fuel cells (PEFCs) causes oxygen transport limitation and pressure drop. This phenomenon, known as "water plugging," is a critical issue for high performance operation. To alleviate this problem, it is necessary to design a cathode channel with optimal structure and to control the surface wettability to promote water removal. In this study, the behavior of liquid water within the cathode channel of an operating PEFC was optically visualized using transparent cells with different channel structures and surface wettabilities. At the same time, the differential pressure between the cathode inlet and outlet and the cell voltage were measured, and the effects of channel structure and wettability on liquid water transport in the cathode channel were investigated. In a serpentine flow channel, both hydrophilic and hydrophobic coatings on the channel sidewall were found to effectively suppress water plugging. In the case of a parallel flow field, hydrophilic treatment of channel sidewall did not cause water plugging on the cathode side. However, hydrophobic treatment impeded the drainage of water accumulated in the channel and lowered cell performance.