In this study, the effect of cathode side gas diffusion layers' porosity on Proton Exchange Membrane Fuel Cells (PEMFC) was modelled. In the model prepared, all the geometric parameters and material characteristics were kept fixed except gas diffusion layer. GDL porosity was between 0.2 and 0.6, and a parametric work was conducted with modifications in the rate of 0.1 per modification. It is understood from the obtained current-voltage graphics that porosity change has different effects on low and high voltage fuel cells. Limit value was 0.63 V, and the highest current levels were obtained when the porosity was 0.3, however, performance of the fuel cell using gas diffusion layer with higher porosities (especially 0.6) increased. The results of the study conducted were shaped based on an experiment study in the literature and the model was verified.