In this paper, a nonlinear state-space model of the 500-W proton exchange membrane (PEM) fuel cell is developed. The open-circuit output voltage of the PEM fuel cell, mass balance, and thermodynamic energy balance inside the PEM fuel cell, voltage losses in the PEM fuel cell, along with the formation of a double layer charge in the PEM fuel cell system are modeled. The model is simulated in MATLAB/Simulink, and the transient response of the PEM fuel cell model over short and long time periods is analyzed. Also, the behavior of the model under the resistive load is studied, and the results are shown.