Deposition of particles in turbulent duct flows is crucial process in a number of industrial and environmental applications. Therefore, in this paper, particle deposition in a horizontal turbulent duct flow is simulated by CFD (computational fluid dynamics) modeling. Dimensionless deposition velocities of particles ranging from 10 to 200mm are predicted by oneway coupling Lagrangian eddy lifetime model combined with a computation model of particle deposition in the turbulent duct flow with a fully developed velocity profiles based on RANS (Reynolds-averaged Navier–Stokes) equations at all three air speeds of 3.0, 5.0 and 7.0ms-1. It is shown that deposition rates (dimensionless deposition velocities) of particles to floors are higher than that to vertical wall and ceiling surfaces. Floor deposition rates increase with dimensionless relaxing time increasing, ceiling deposition rates decrease to zero with dimensionless relaxing time increasing, and wall deposition rates show the increase first and then a subtle decrease with dimensionless relaxing time increase at different air speeds. Wall and ceiling deposition rates increase with air speed increase at the same dimensionless relaxing time. Floor deposition rates decrease with air speed increase at the same dimensionless relaxing time. The simulation results are expected to be helpful for evaluating particle deposition rates in some environmental and industrial square ducts.