A model was proposed for simulation of a particle formation stage in the dispersion polymerization of styrene in ethanol. The role of poly(N-vinylpyrrolidone) molecules grafted by a polystyrene oligomer (PVP-g-PS) was taken into consideration for the particle stabilization. The model was derived based on the following mechanism: (i) polystyrene molecules whose degree of polymerization is greater than j(cr) precipitate and form particles, (ii) particle aggregation due to the Brownian diffusion and the shear stress of the fluid continues until the amount of the PVP-g-PS molecules is enough to cover all the particle surface, and (iii) particles do not aggregate with each other once the entrire particle surface is covered with the PVP-g-PS molecules. The theoretical particle concentrations calculated using the present model under various monomer concentrations and stirring speeds were in good agreement with the experimental data.