Application of polymer nanoparticles has progressively broadened. There is now increasing interest in smaller polymer nanoparticles for use in organic solar cells and drug delivery systems. Unfortunately, it is difficult to control the particle size below 50 nm with conventional synthesis methods. Our previously proposed "two-step nanoprecipitation method" overcomes this problem. An oil-in-water emulsion is first formed from polymer solution and deionized water without using surfactant; it is then injected into ethanol to form particles. The particle formation mechanism in this method has now been investigated, and an interesting phenomenon was discovered: the injected droplets instantaneously split into nanosize droplets with a size of 100-150 nm. The splitting was very effective, and the formed nanosize droplets were virtually monodisperse. This occurred only for a mixture composition in which the surface tension of the poor solvent mixture (water and ethanol) equaled that of the good solvent. This composition also resulted in formation of the smallest particles. By adjusting the conditions, we were able to synthesize extremely small nanoparticles (∼5 nm) of poly(3-hexylthiophene). This method has the potential to synthesize nanoparticles composed of other types of materials, such as nonconductive polymers and small molecules.