The work presented here reports on the numerical simulation of an electrostatic powder coating process that uses a commercial computational fluid dynamic code, FLUENT v6.1. The purpose of this study was to understand the gas and particle flow fields inside a coating booth under given operating conditions and the effect of particle sizes on its trajectories and the final coating quality. The air and powder particle flows in a coating booth were modeled as a three-dimensional turbulent continuous gas flow with solid particles as a discrete phase. The continuous gas flow was calculated by solving Navier-Stokes equations including the standard k-epsilon turbulence model with non-equilibrium wall function and the discrete phase was modeled based on the Langrangian approach. Since the solid phase volumetric fraction was less than 0.1%, the effect of particle-particle interaction on particle trajectories was not taken into account. In addition to drag force and gravity, the electrostatic force including the effect of space charge due to the free ions was considered in the equation of motion and implemented using user defined scalars and functions. The governing equations were solved using the second order upwind scheme. Information was provided on the particle trajectories with respect to the particle diameters that could be used to develop suitable operating conditions for the use of fine powders in a powder coating process. (c) 2006 Elsevier B.V. All rights reserved.