A coalescence mechanism for the coarsening behavior of polymer blends during a quiescent annealing process is proposed, in which the effect of different particle sizes is taken into consideration. All existing forces, such as thermal agitation, van der Waals attraction, interfacial tension, and viscous resistance, control the coarsening process. The evolution of the particle size and its distribution are modeled with a particle population balance equation. The experimental investigation has been made with the transmission electron microscopy observation of a polystyrene/poly( methyl methacrylate) blend during the annealing process. The model predictions are in good agreement with our experiment and other authors' work. The relative effects of system parameters such as volume fraction, interfacial tension, and viscosity on the particle size distribution are also discussed.