A full three-dimensional, multi-phase computational fluid dynamics model of a PEM fuel cell has been developed. The parametric study using this model has been performed and discussed in detail. Optimization study of a PEM fuel cell performance has been performed. The study quantifies and analyses the impact of operating, design, and material parameters on fuel cell performance and get an optimal design for PEM fuel cells to generate maximum power. To generate maximum power, the results show that the cell must be operate at higher cell operating temperature, higher cell operating pressure, higher stoichiometric flow ratio, and must have higher GDL porosity, higher GDL thermal conductivity, narrower gases channels, and thinner membrane. At these optimum conditions, the result shows that the total displacement and the degree of the deformation inside the MEA were decreased. However, the Miss stress in the membrane was increased due to higher cell operating temperature.