It is important to elucidate the transient characteristics of polymer electrolyte fuel cells (PEFC), especially when PEFC is applied to relatively small-scale power applications where it will be subjected to a wide range of loads, and may have frequent starts and stops. In addition, the water management problem, which is represented by flooding in cell and drying in proton exchange membrane (PEM), is another issue to address. The flooding is caused by liquid water accumulated in GDL and/or flow channel; the liquid water hinders mass transfer of gases to and from active layers; it can lead to rapid deterioration of cell performance. And the water management relates to the transient response of PEFC frequently. Based on these issues we wrote a numerical simulation program for unit-PEFC, which can simulate the successive events of vapor condensation, liquid saturation growth, corresponding to the dynamic change of cell voltage. We formulated mass, momentum and energy conservation equation with equivalent electric circuit; we discretized and numerically solved them. As for the gas/liquid two-phase flow formulation in GDL, we utilized multi-phase mixture (M2) model. As for the multi-component diffusion formulation, we utilized Stefan-Maxwell equation. Using the program, we simulated the transient response to rapid increase of load current. When the current density changed from 0.5 A/cm2 to 1.0 A/cm2 instantaneously, cell voltage (Vcell) changed in the following manner. Just after the change of current, Vcell decreased instantaneously corresponding to IR resistance and decreased again in 10-1s time-scale with the re-distribution of oxygen and with the charge of electric double layer capacitor. Then Vcell increased slightly in 101s time-scale with PEM wetting. Finally, Vcell decreased in 10 2-103 s time-scale with the development of liquid saturation in GDL.