Spatial scale characteristics of particle clusters arc; investigated by using the flow field data obtained from tithe-dependent three-dimensional numerical simulations. Eulerian/Lagrangian approach with two-way coupling is adopted and individual particle-particle collisions are taken into account by using the hard-sphere deterministic model.. More than sixteen million particles are tracked in the maximum case. The results show that the particle cluster consists of multiple-spatial scale components and a low wave number, hence the large-scale structure, is dominant. Three-dimensional structure reconstructed from the low-pass filtered data enables us to investigate the essential dynamics of particle clusters in detail. From the domain size-dependency study, it is found that the structure of particle cluster is sensitive to the horizontal domain size when the periodic boundary condition is used. (c) 2007 Elsevier B.V. All rights reserved.