Microencapsulation of sulfamethizole by using Eudragit RS was investigated by the evaporation process in water phase. Some metal stearates were used to reduce the flocculation of microcapsules in the preparation. However, the microcapsules just after separated from the water phase were swollen. The minimum swelling ratio was obtained in the case of aluminium tristearate. Therefore, the use of aluminium tristearate as an additive was found to be most suitable for the microencapsulation by using Eudragit RS. Furthermore, an optimum amount added was found to exist for each of the metal stearates in this experiment. The microcapsules obtained after drying under reduced pressure were uniform and free-flowing particles. The dissolution rates of sulfamethizole from these microcapsules were relatively reduced compared to that from sulfamethizole powders.