The method for simulating the time-evolution of the particle size distribution of polymer particles generated in the O/W microemulsion polymerization of styrene initiated by a water-soluble initiator is proposed based on the following kinetic model: The radicals generated in the aqueous phase enter the micelles, propagate therein and transform them into new polymer particles with (1) negligible bimolecular termination in the aqueous phase, and with (2) negligible entry of the second radical into the preformed polymer particles, and hence, (3) propagation of growing radicals stops only by a chain transfer reaction to monomer, (4) growth of polymer particles stops only by the desorption of the radicals generated by chain transfer out of the particles, and (5) the radicals produced by chain transfer may desorb from the particle or initiate propagation in the same particle. The average diameter calculated from the simulated particle size distribution agrees well with that measured experimentally by dynamic light scattering. The simulated particle size distribution agrees with that measured experimentally with a transmission electron microscope.