Discrete particle simulation has been recognized as a useful numerical technique to elucidate the fundamentals of particle fluid flow, in particular, gas fluidization. In general, it is achieved by combining discrete element method (DEM) for solid flow with computational fluid dynamics (CFD) for continuum gas. Limited by computational capability, such studies are mainly conducted using a two-dimensional (2D) bed in which particles are treated as discs or a pseudo-three-dimensional (3D) bed where the bed thickness is the same as the particle diameter. The loss of one-dimensional motion may significantly affect the flow and related behavior. This paper presents a numerical study of the effect of the front and rear walls of a fluidized bed on the mixing and segregation behavior of particle mixtures. Particle diameters used for simulation are 1 mm for flotsam and 2 mm for jetsam, respectively. Numerical simulations are conducted for two beds of different thicknesses, fluidized at different gas velocities. One, referred to as the 2D case, shows the bed thickness is 1.1 times the thickness of the particle diameter of jetsam with the front and rear walls supporting the particles; the other, referred to as the 3D case, shows the bed thickness is 4.05 times the particle diameter of jetsam with the application of periodical boundary conditions to the front and rear walls which simulate the 3D motion of particles in the bed. Comparison conducted in terms of solid flow patterns shows that both 2D and 3D simulations can capture the key features of the mixing/segregation process, but the transient segregation occurs at different velocity ranges. Further comparison is conducted on the basis of microdynamic variables, such as mixing kinetics, interactions between particles and fluid, and between particles, and particle contact numbers. These quantitative comparisons demonstrate a significant difference between 2D and 3D simulations, which indicates the necessity of 3D simulation for engineering application.