Air and particle motions in a three-dimensional turbulent fluidized bed which contains more than 24 million small particles (D-p=310 mu m) are numerically simulated. In this study, a stochastic scheme based on Direct Simulation of the Monte Carlo (D. S. M. C.) method was introduced into the calculation of particle collisions in order to treat a large number of particles. The locally averaged three-dimensional Navier-Stokes equations and Lagrangian type particle motion equations, where the drag and lift forces acting on particles, the collisions between particles and the mutual interactions between air and particles are taken into account, was simultaneously solved. Collisions between particles were calculated by using the hard sphere model (this means that a collision occurs between only two particles). Calculated results using D. S. M. C. method are in fairly good agreement with the experimental results of flow visualizations and air and particle turbulent characteristics. Calculated results described the formation of particle clusters well. Both calculated and experimental results quantitatively showed that turbulent motions of air and particles in a fluidized bed are remarkably enhanced by their mutual interactions.