Nonideal mixing behavior of solid particles in a fluidized bed for catalytic polymerization is examined for its effects on the particle size distribution (PSD). It is shown that the assumption of perfect mixing for the solid is quite adequate for the PSD under typical operating conditions. However, the assumption leads to erroneous PSD when the size of the particles removed from the bed is relatively small. Analytic expressions of the PSD are obtained that take into account both diffusion and catalyst deactivation. The diffusion effects are shown to be fairly significant. A simple relationship is derived for an estimate of the average particle size.