Electromagnetic waves are composed of electric and magnetic field components and, propagate in space or material media. When an electromagnetic wave hits a surface, depending on the electrical characteristic of the surface, some parts of the electromagnetic wave are scattered while some parts are transmitted.  A Frequency Selective Surface (FSS) changes its reflection and transmission characteristic depending on the frequency of the incident electromagnetic wave. By this properties the FSS structures operate as a fitler for electromagnetic waves. In this study, a band-stop FSS was designed for a 5GHz frequency band.  In FSS design; parameters such as FSS geometry, its period, the element type used, range between the arrays,  parameters of the medium, angle of incidence and polarization of the incident wave are important.  The shielding effect of the FSS in a 5GHz frequency band was examined by using Green Functions and the Method of Moments (MoM). In fitler design square type patch elements were used. The FSS design was also tested under vertical polarization for different number and length square patches. Best shielding effect was obtained under vertical polarization with  and  for a FSS design having 1 square pacth. Both CPU and GPU were also tested seperately in  linear equation system solution. For a  “A” matrix anda  “b” vector solution took approximately 0.19ms on GPU and 6.22 ms on CPU. It was observed that the GPU system speed increased approximately 33 times and provide more efficient solutions according to CPU.