Unseeded emulsion polymerization of methyl methacrylate was carried out at 50-degrees-C using sodium lauryl sulfate as emulsifier and potassium persulfate as initiator, respectively, to clarify the effects of emulsifier, initiator and monomer concentrations initially charged on the kinetic features of the polymerization such as the number of polymer particles produced and the course of polymerization. To describe the observed kinetic features, a quantitative kinetic model for particle nucleation and growth is proposed on the basis of the assumption that polymer particles are generated from emulsifier micelles. Furthermore, the rate coefficients for radical termination and propagation in the polymer particles are empirically correlated as a function of the weight fraction of polymer in the polymer particles. It is shown that by using these empirical rate coefficients and the proposed kinetic model, the observed monomer conversion versus time histories can be successfully simulated up to almost the end of polymerization, when the emulsifier concentration is higher than the critical micelle concentration.