A mathematical model based on the distinct element method (DEM) was developed to investigate the hydrodynamics in a gas-solid down-flow circulating fluidized bed reactor (downer). The models consist of the equations of particle motion and fluid motion. The contact force is calculated by using the analogy of a spring, dashpot, and friction slider. Simulation results show that the radial solids holdup and particle velocity profiles are uniform in the core region. Near the wall, the solids holdup is higher with lower particle velocity. An increase in the particle size decreases the solids holdup and increases the particle velocity. The solids holdup decreases with superficial gas velocity but increases with solids circulation rate. Particle velocity increases with gas velocity and solids circulation rate. The solids holdup and particle velocity are almost uniform along the height of the downer except near the distributor. The hydrodynamic behavior from this simulation showed trends similar to those of the experimental results. The results obtained from this model fit better with the experimental results than Kimm's and Bolkan's models do.