Preferential solvation of polymer molecules and strong EPD-EPA (EPD, electron pair donor; EPA, electron pair acceptor) interaction between solvent and nonsolvent molecules were found to be of great significance in the fabrication of two kinds of aromatic polyimide (AP) nanoparticles. Surfactant free yet stable AP nanoparticles were prepared using a liquid-liquid phase separation method. The stability of the AP nanoparticles can be achieved by the solvation multilayer resulting from a solvation stabilization chain in the form of nonsolvent -> solvent -> AP (a -> b denotes that component b is solvated by component a). The significance of this stabilization chain was identified by many comparative experiments using different types of molecular probes. On the other hand, the formation of AP nanoparticles was found to be governed by a nucleation process and therefore the particle size is controlled by the nucleation rate. A very high level of supersaturation can be attained under the intensive local motions induced by ultrasound, resulting in a very high nucleation rate. This effect was found to be extremely useful in the fabrication of sub-50 nm AP nanoparticles.