Air and particle motions in a 3-dimensional circulating fluidized bed which contains about 380million particles (diameter, D-p = 115 mu m) were numerically simulated. A stochastic scheme based on the DSMC method was introduced into the calculation of particle collisions supposed to be two body collisions to treat a large number of particles. The locally averaged 3-dimensional Navier-Stokes equations and Lagrangian particle motion equations, in which the drag and the lift forces acting on particles, collision between particles and the mutual interaction between air and particles were taken into account, were simultaneously solved. The subgrid scale flows were modeled in Navier Stokes equations using LES in which the effect of the particle existence on the subgrid scale flows were considered. The good agreement between the present calculated results and experimental data in the circulating fluidized bed shows the applicability of the present simulation method to various gas and particle flows for examples in particulate matter pneumatic conveyors, high particle concentration cyclone separators, high mass mixture gas-solid jets, etc. Calculated results clearly present the particle clusters of which shape and size were in good agreement with the experimental results. These indicate that the cluster formation mechanisms are the non-clastic collision among particles and the interaction between the air and particle flows. Calculated results of flow characteristics which agree with experimental results show that the existence of small particlcs remarkably enhances the turbulent motions of air and particles in a circulating fluidized bed.