Numerical simulations based on three-dimensional discrete element model (DEM) are conducted for mono-disperse, binary and ternary systems of particles in a fluidized bed. Fluid drag force acting on each particle depending on its size and relative velocity is assigned. The drag coefficient corresponding to Ergun’s correlation is applied to the system of fluidized bed with particle size ratios of 1:1 for the mono-disperse system, 1:1.2, 1:1.4 and 1:2 for the binary system and 1:1.33:2 for the ternary system by keeping total volume and surface area of the particles constant. Results indicated that a reasonable estimation of drag force based on individual particle diameters as compared to that of the mean diameter of the particles is achieved in the fluid cells. The total translational kinetic energy of the particles is found to increase as the particle size ratio increases, suggesting an enhanced momentum transfer in polydisperse particle systems. Systems with wide particle size distribution exhibited higher particle velocities around bubbles, resulting in faster bubble growth and its subsequent rise through the fluidized bed. Numerical simulations based on three-dimensional discrete element models (DEM) are conducted for mono-disperse, binary and ternary systems of particles in a fluidized bed. Fluid drag force acting on each particle depending on its size and relative velocity is assigned. coefficient corresponding to Ergun’s correlation is applied to the system of fluidized bed with particle size ratios of 1: 1 for the mono-disperse system, 1: 1.2, 1: 1.4 and 1: 2 for the binary system and 1: 1.33: 2 for the ternary system by keeping total volume and surface area of ​​the particles constant. Results indicated that a reasonable estimation of drag force based on individual particle diameters as compared to that of the mean diameter of the particles is achieved in the fluid cells. kinetic energy of the particles is found to increase as the particle size ratio increases, suggesting an enhanced momentum transfer in polydisperse particle systems. Systems with wide particle size distrib uncemented examined higher particle velocities around bubbles, resulting in faster bubble growth and its subsequent rise through the fluidized bed.