Polypropylene particles were prepared by a thermally induced phase separation. The particle formation occurred by the nucleation and growth mechanism in the metastable region. The particle growth was followed by the dynamic light scattering measurement. Although the growth rates depended on the experimental conditions such as the polymer concentration and temperature, the particle diameter d increased with time t in accordance with the relation of d(3) cc t in all cases. Detailed consideration on the slope values indicated that the particles grew by the coalescence mechanism. The particle size distribution, which was obtained by SEM observation, became broader when the polymer concentration was higher.