The styrene microemulsion with high monomer content was stabilized by an emulsifier with a Y structure. When the microemulsion polymerization was initiated with azobisisobutylronitrile (AIBN) or benzoyl peroxide (BPO), a plateau of polymerization rate was observed at low initiator and high monomer content. Zn the plateau, it was the number of growing polymer particles that was kept constant, instead of the number of total polymer particles. The length of the plateau of R-p is reduced with the increase of initiator concentration and the decrease of monomer concentration, and even disappears at very low monomer content and very high initiator concentration. Under the similar condition, the polymerization rate and the molecular weight of polystyrene produced are both lower when the initiator is BPO.