► Water droplet Dynamics at GDL surface are analyzed both theoretically and numerically. ► Drag forces on a droplet are examined in both entrance and fully developed regions. ► Droplet deformation due to pressure variation is analyzed. ► The droplet detachment velocity at GDL surface is analytically obtained. ► Relation between the Weber and Reynolds numbers at the droplet detachment velocity is developed. Water management is critical to achieving/maintaining high performance of polymer electrolyte fuel cells (PEFCs); and elucidating the dynamic behavior of liquid water droplets in a PEFC channel is essential to water management. In this work, the dynamics of liquid water droplets in a single PEFC gas flow channel is investigated through theoretical and numerical analyses. Forces on water droplet, droplet deformation and detachment are examined. The pressure and viscous drags are computed and compared at different flow regimes (which exhibit different droplet-dynamic scenarios) such as that in the entrance and fully developed flow regions. The expression for describing droplet shape change is derived, and it is found that the droplet can deform significantly at high gas-flow rates and when the droplet is relatively large (relative to the channel dimension). The detachment velocity is analyzed by comparing the wall adhesion and drag forces, and an expression relating the Weber number to the Reynolds number using the detachment velocity is developed. Follow-on work is also briefly discussed.