A coalescence mechanism for the coarsening behavior of polymer blends during a quiescent annealing process is proposed. It suggests that the distribution state of dispersed particles in a matrix obeys a nearest-neighbor distribution function in multibody systems. Both drainage and merge stages are included in the coalescence. All existing forces, such as thermal agitation, van der Waals attraction, interfacial tension, and viscous resistance, take control of the coarsening process. A dynamic equation is used to compute the evolution of dispersed particles. An experimental investigation has been made with the time-resolved light scattering measurement of a polystyrene/poly(methyl methacrylate) blend during an 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 are also discussed.