Many industrial operations, such as liquid-liquid mixing and multi-phase reaction, involve the for- mation of stirred dispersions of immiscible liquids. In a polymerization reactor to produce incompatible polymer mixture, the prediction of average drop size is one of the most important factors that affect the productivity and product quality In such systems drop sizes are determined by the condition of flow ranges. Oil-in-oil dispersions such as polymeric emulsion often occur within the viscous shear subrange in turbulent agitation, but few attempts have been made to predict the drop size. In this study, a mechanistic model based on hydrodynamic force balance is suggested to predict the average drop size in polymeric emulsion. The agreement between the model and the data of high impact polystyrene was quite satisfactory for the prediction of the average particle size of rubber dispersed phase.