We have fabricated organic nanocrystals of low-molecular-weight aromatic compounds such as perylene and anthracene using the so-called reprecipitation method, in which a dilute solution of the compound prepared by water-soluble solvent, i.e., good solvent, is injected into vigorously stirred water as a poor medium. The resulting nanocrystals are dispersed stably in water. It was found that the average size of perylene nanocrystals formed, as evaluated by dynamic light scattering technique, decreased as the dielectric constant of the good solvent increased, while the size of the anthracene nanocrystals increased with increasing dielectric constant due to association in the aqueous dispersion liquid. The influence of solvent polarity and solubility on the nanocrystallization process in the reprecipitation method is discussed in detail. (c) 2006 Elsevier B.V. All rights reserved.