A polymer electrolyte membrane fuel cell is investigated in this study to assess its efficiency. In this regard, various operating conditions such as cell temperature, pressure, reactant/product flow rates and humidity affects are investigated analytically, and their interrelationships are discussed. The stack water management, mass transport phenomenon, ionic and electrical conductivity are also evaluated. The results are experimentally verified using a polymer electrolyte membrane fuel cell with an active surface area of 100 centimeters square. The membrane electrode assembly consists of Nafion (R) HP membrane. Also, AvCarb EP40 gas diffusion layers with 200 mu m thicknesses are used. Results confirm that the overall stack efficiency can increase remarkably with the optimization of its operating parameters. The highest efficiencies are achieved around 100% humidity ratio of reactants at both cathode and anode. While high operating pressures improves individual cell efficiency, there are contradictory concerns at the stack level such as parasitic loads, losses, leakages and manufacturing costs.