The performance of Electrostatic precipitator (ESP) is significantly affected by its complex flow distribution arising as a result of its complex inside geometry. In the present study the gas flow through a lab-scale ESP is modelled numerically using computational fluid dynamics (CFD) technique to optimize the flow distribution inside the ESP. CFD code FLUENT is used to carry out the computations. Numerical calculations for the gas flow are carried out by solving the Reynolds-averaged Navier-Stokes equations coupled with the realizable K-ε turbulence model equations. In the simulation the perforated plates, used inside the ESP, are modelled as thin porous media of finite thickness with directional permeability. The results of the simulation are discussed and compared with laboratory measured data. The model was used to simulate an optimized flow inside the ESP. The model developed could be used as a novel tool to predict the effect of possible modifications made to the ESP design on the flow pattern.