Soap-free emulsion polymerization is an environmentally desirable process that can produce highly monodisperse polymer particles with high purity. In the polymerization, knowledge on the mechanism of particle growth is important for size control of the polymer particles. Experimental and theoretical study is performed on the self-sharpening of particle size distributions appearing with particle growth in soap-free emulsion polymerization. The dependence of particle growth rate on particle size was determined with two experimental methods of the seeded growth and non-seeded polymerizations of styrene. In the seeded growth polymerization, polymer particles with two different sizes were used to measure relative growth rate of the particles in the same reaction field. In the non-seeded polymerization, the relative growth rates were measured from time-variation of a particle size distribution during the particle growth. Two initiators, potassium persulfate and 2,2'-azobis[N-(2-carboxyethyl)-2methylpropionamidine] hydrate at various pH, were employed to conduct the experiments in a wide range of particle sizes 0.1-2 mu m. Both the experiments showed the same tendency of the size-dependence, which indicated that the self-sharpening becomes weak with an increase in particle size. This tendency is quantitatively explained by the particle growth mechanism that considers the gel effect of radical polymerization and the interfacial energy of monomer-swollen polymer particles. (C) 2015 Elsevier Ltd. All rights reserved.