The perforated plates are commonly used for gas flow control in the wide-angle diffusers of electrostatic precipitators of large power plants. Many studies dealt with the investigation of the effects of the perforated plate’s geometry on flow parameters in the cases where incoming flow is perpendicular to the plate and the plate is covering the whole cross-section of the flowing channel. These results are partially applicable in cases where flow is inclined on the plate or when the plate is not occupying the whole cross-section of the channel. The subject of this work is a numerical investigation of flow through the rectangular channel with a perforated plate in various positions in the cross-section of the channel. The aim was to investigate the effect of the plate position on the flow. The perforated plates were modeled as thin porous media of finite thickness by using the directional loss model. Numerical experiments are carried out by using CFD software ANSYS CFX. Results of pressure drop and velocity distribution behind the plate are compared to the results of CFD simulation of the full 3-D plate model. In order to obtain a reasonable agreement both of the pressure drop and velocity distribution behind the plate when using a simplified thin porous plate model, the value of streamwise permeability of the plate had to be adjusted. The level of adjustments has determined iteratively and it depends on the plate position in the channel’s cross-section.