Particle clusters formed in gas-solid flows in risers of circulation fluidized beds or in pipes of pneumatic transport system have much influence on various kinds of transport phenomena. In the present study, the structure of the flow with clusters is investigated numerically. The Lagrangian/Eulerian approach and the two-way coupling are applied because of the large effect of the solid phase onthe gas flow. The gas flow was solved based on the N-S equation. the motion of individual particles is calculated taking into account particle-particle collisions by a deterministic method. The effects of distribution of particle diameter on the cluster structure is studied. The calculated results are compared with experimental ones measured in a vertical duct with respect to the fluctuation of local solid volume fraction and the spatial structure of the clusters.