To simulate cluster formation in dense two-phase flow, a multi-lattice deterministic trajectory (MLDT) model is developed. The actual inter-particle collision and particle motion are treated by a Lagrangian trajectory model with three sets of lattices to reduce computation time. Initially, the MLDT model is used to simulate dense gas-particle flow in a vertical channel for validation with available simulated and experimental data. Using the MLDT model, effects of particle properties and flow parameters on cluster formation are studied. It is shown that cluster formation is enhanced by the reduction of particle restitution coefficient and increase of original particle volume fraction. (c) 2005 Elsevier B.V. All rights reserved.