Based on the assumption that an equilibrium particle size of dispersed phase will be reached when the breaking-down rate and the coalescence rate are balanced, a theoretical expression was obtained. The theory showed qualitatively that the equilibrium particle size becomes smaller when (1) the stress field is increased, (2) the interfacial tension between matrix and dispersed phase becomes smaller, and (3) the concentration of dispersed phase decreases. Qualitative verification of the theory was obtained by experimental examination of the NR-EPM blend system. In practice, in order to obtain a small particle size in a short time at above 20% volume fraction, the matching of rheological properties of the matrix and the dispersed phase is desirable. On changing from internal mixer to mill, the temperature became one of the most influential factors that control the particle size. Future work, such as the quantitative value of interfacial tension as a function of temperature, and macroscopic breaking energy measurement, etc., is necessary to confirm the theory quantitatively.