Gas/particle flow behavior in the riser section of a circulating fluidized bed (CFB) was simulated using a computational fluid dynamics (CFD) package by Fluent. Fluid catalytic cracking (FCC) particles and air were used as the solid and gas phases, respectively. A two-dimensional, transient and isothermal flow was simulated for the continuous phase lair) and the dispersed phase (solid particles). Conservation equations of mass and momentum for each phase were solved using the finite volume numerical technique. This approach treats each phase separately, and the link between the gas and particle phases is through drag, turbulence, or energy dissipation due to particle fluctuation. Gas and particle flow profiles were obtained for velocity, volume fraction, pressure, and turbulence parameters for each phase. The computational values agreed reasonably well with the available experimental results. Our computational results showed that the inlet and outlet design have significant effects on the overall gas and solid flow patterns and cluster formations in the riser. However, the effect of the initial condition tended to disappear after some time. The main frequencies of oscillations of the system were obtained in different regions of the riser. These frequencies are important in comparing the computational results with the available time-averaged experimental data.