Digital simulation is the effective method to have a knowledge on the screening efficiency of complex multi-deck separation system and reliable tool for optimizing the process. Controlling parameter of the screening process is the screen aperture. The paper presents results of digital experiments performed on two 3-deck screens. The screening efficiency of the process along the surface of three-deck screen device is analyzed in the paper. Each deck of a screen is assumed as the subsequent system of 6 screens of the equal length and different size aperture. Simulation of the process performed gave the answer how the screened material is distributed by the mass and grain size as well as how the screening efficiency is distributed depend on feed load and size distribution or parameters of the screens. The total result of the process was evaluated by process quality indicator. Calculations are based on the mathematical model of screening as a function geometric and dynamic parameters of the screen.