The emulsion polymerization of methyl methacrylate (MMA) in concentrations of emulsifiers below their critical micelle concentrations (CMCs) initiated by K2S2O8 (KPS) was studied. It was observed that the initiator concentration has little effect on both polymerization rate and particle size. However, the polymerization rate is faster and particle size is smaller obviously when decreasing the ratio of the water/monomer or increasing the temperature of polymerization or the amount of the emulsifier. In the range of a 200-400 rpm stirring speed, the polymerization rate is almost unchanged although the particle size becomes larger with increase in the stirring speed. The monodisperse particle (size about 100-200 nm) can be obtained using this process. The mechanism of emulsion polymerization of MMA in the emulsifier concentration below its CMC is discussed.