A new two-stage mathematical model of ultrasonically initiated emulsion polymerization, only acceleration and deceleration stages, was proposed in this paper. This model took into account homogeneous, micelle and monomer droplet nucleation mechanisms. According to this model, the reason of constant velocity stage disappeared is due to very abundant small size monomer droplets nucleation after the surfactant exhausted. Moreover, for St and MMA, the model predicts that as the ultrasonic intensity increases the monomer conversion and polymerization rate at stage I will rapidly increase while the ending time of stage I rapidly decrease. However, as the monomer content increases the monomer conversion and polymerization rate at stage I will rapidly decrease while the ending time of stage I rapidly increase. For the two monomers mentioned above, the model predictions are both very excellent agreement with the experimental data from the literatures. (c) 2006 Elsevier Ltd. All rights reserved.