This paper reports on a research project that studies the effect of particle size on the coating efficiency and coating uniformity in-a powder coating system using the computational fluid dynamics as a modelling tool. The numerical simulations are conducted for different particle sizes with different distances between the spray gun and the coating part and different positions of the powder spray gun pattern adjuster sleeve (PAS). This study can provide detailed information on air flow pattern and particle trajectories inside the powder coating booth, and the coating film thickness on the coated part as well as the particle transfer efficiency (PTE). In numerical simulations, the air flow field is obtained by solving three-dimensional Navier-Stokes equations with standard kappa-epsilon turbulence model and non-equilibrium wall function. The second phase, the coating powder, consists of spherical particles that are dispersed in the continuous phase the air. In addition to solving transport equations for the air the trajectories of the particles are calculated by solving the particle motion equations using the Lagrangian method. It is assumed that particle-particle interaction can be neglected. The electrostatic field is modelled by solving the Laplace equation.