Three-dimensional (3D) complex flow-fields of proton exchange membrane (PEM) fuel cells have attracted much attention owing to their excellent liquid water management and mass transport. However, due to their complex flow structure, PEMFCs with 3D complex flow-fields suffer from large pressure drops (> 0.1 bar) and hence large density variations along the flow direction, especially at high current density operations. In this work, the effect of gas density variation due to the frictional pressure loss is considered in the current three-dimensional computational model using the multi-phase mixture (M 2 ) formulation, in order to elucidate the effect of frictional loss on cell performance. The current work shows that the gas density drop in flow-fields can be significant at high current densities (20% at 3.0 A cm -2 and 30% at 4.0 A cm -2 ) and causes gas flow expansion, resulting in better liquid water management in flow-fields and gas diffusion layers (GDL) due to the gradually increasing gaseous viscous force along the flow direction. However, it is also pointed out that the gas density drop in cathode flow-fields results in cell performance loss due to lower oxygen concentrations (15mV voltage loss at 0.5 bar pressure drop, 60mV voltage loss at 0.78 bar pressure drop).