Abstarct: In this research, the process of sieving granular materials by vibrating three-panel screen with the discrete element method has been simulated. The laws of collision and inter particle forces are presented and the Hertz-Mindlin law is used to model the force between particles and boundaries. The screened particles are spherical in shape and the screen aperture are square and rectangular. During the simulation, the differential equations governing the motion of spherical particles are solved numerically and their behavior is predicted. The software used was open source under the Linux operating system, the geometric dimensions of the three-plane screen and the physical parameters of the feed were determined. After performing ninety-four simulations, the effect of various parameters such as vibration amplitude, vibration frequency and aperture shape on the screening efficiency were investigated and the relevant results were reported. Screen efficiencies with square and rectangular apertures in different amplitudes and frequencies are compared and the results obtained with laboratory tests and simulations are compared quantitatively and qualitatively and the difference results were less than 5%. At low vibration amplitude and high vibration frequency, the square shaped screen has better efficiency than the rectangular sized screen, but with increasing vibration amplitude, the rectangular sized screen efficiency is up to 10% higher than the square sized screen.