The importance of fossil fuels has increased with the use of steam energy in industry. But especially in recent days, the problem of global warming is being felt seriously in the world. The most important reason for this problem is harmful emissions caused by burning fossil fuels. The world has turned to various alternative energy sources in order to eliminate this problem. Interest in PEM fuel cells, which are among alternative the energy converters, has been increasing. In this study, the effect of stoichiometry ratio on the performance of two-cell PEM fuel cell stack was analyzed numerically. Two fuel cells each with an active area of 5.4 cm2 were connected in series, and the gas flow channels were determined in parallel and stacked. Numerical analysis was performed with five different stoichiometry ratios (1.5, 2, 2.5, 3, and 3.5) at various cell voltage values (1 V, 1.2 V, 1.4 V, 1.6 V, 1.8 V). Polarization and power- current density curves were obtained for five different stoichiometry ratio values. In addition, in order to see the effect of the stoichiometry ratio on a constant voltage, temperature distribution of the membrane, the anode and the cathode mass fraction distributions formed by four different stoichiometric ratios at 1 Volt, where the maximum amount of power is obtained. With this study, it was observed that current density increased with the improvement in the stoichiometric ratio and the amount of this increment continued to decrease. The highest power value was obtained at the stoichiometric ratio of 3.5 and 1 V potential difference value. However, with the increasing stoichiometric ratio, it was concluded that hydrogen and oxygen distributions became more homogeneous in the anode and cathode channels while a limited increase in membrane temperature occurred