Three-dimensional numerical investigation of a low speed particle-laden turbulent flow over a backwardfacing step has been carried out. An assumption of incompressibility of the flow is used due to low Mach number of the flow. The gas phase is performed by Large Eddy Simulation (LES) and the particle phase is solved by a Lagrangian particle tracking model. The simulation results such as mean streamwise velocities and fluctuation velocities for the both phase are validated by experimental results performed by Fessler and Eaton (1999) . Reynolds number of the gas phase over the backward-facing step with an expansion ratio of 5:3 is 18,400, based on the maximum inlet velocity and step height. The flow is considered as dilute. Hence a one-way coupling method is applied, in which we only consider the effect of fluid on the particle. Particle-particle collisions are also neglected. The success of simulation in predicting a particle-laden turbulent flow using LES and Lagrangian trajectory model provides a numerical basis for revisiting the gas-particle correlations models. Four second-order closure models for gas-particles covariance are evaluated in the present study. A modified better gas-particle covariance model is proposed in this paper.