Abstract: AMPS(2-acrylamido-2-methylpropanesulfonicacid)/MMA(methyl methacrylate) copolymer microspheres were prepared with the solution containing MMA and AMPS without initiator and emulsifier by ultrasonic irradiation emulsifier-free emulsion polymerization. The effect of ultrasonic time on monomer conversion was investigated, the samples were characterized by FTIR, TGA, DSC, TEM and Zeta-sizer and the initiation mechanism of copolymerization was studied as well. The results show that the microspheres possessed a good dispersity, a uniform size with the diameter of 0.77μm and an excellent thermal stability.