Polymer microcapsule is a kind of functional material with unique structure. Study on the preparation methods is growing obviously. An efficient and environmentally friendly method for the preparation of polymer microcapsules is presented in this article. PMMA microcapsules were prepared by ultrasonically initiated miniemulsion polymerization phase separation method. The morphology, size and its distribution of PMMA microcapsules were investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM) and dynamic light scattering (DLS). And the polymerization kinetics was also discussed. The results show that PMMA microcapsules with size of ca. 100 nm can be obtained when the amount of coemulsifier is 3 times of that of the emulsifier, the amount of monomer is 3 wt% of the system and the reaction temperature is 80°C. In all experiments, N2 was bubbled to protect the reaction system.