A seeded soapless emulsion polymerization was carried out with poly(methyl methacrylate) (PMMA) as seeds, styrene as monomers, and potassium persulfate (K2S2O8) as the initiator to synthesize the PMMA/polystyrene (PS) composite latex. The morphology of the latex particles was observed by transmission electron microscopy (TEM). It showed a core-shell structure. A core-shell kinetic model was proposed for the seeded emusion polymerization, in which the thickness of the shell was not constant. An increase of the conversion would increase the thickness of the shell. The entire course of polymerization could be divided into three regions : In the first region, the propagation rate constant (K-p) and termination rate constant (K-t) were kept constant at constant temperature. The kinetic data showed that the square root of polymer yield (W-p(1/2)) was proportional to the reaction time. In the second region, the gel effect was considered and the termination rate constant (K-t) was empirically modified. The K-t would decrease with increasing the conversion. In the third region, both the gel effect and the glassy effect were considered; the propagation rate constant was also empirically modified. The prediction on the conversion and the number-average molecular weight of polymers during the seeded emusion of polymerization on the basis of our core-shell kinetic model fitted well with the experimental data.