We found a simple method to prepare polymer nanoparticles by evaporation of solvent from a polymer solution. This method is applicable to prepare of nanoparticles of many kinds of functional materials (eg. engineering plastics, fluorescence polymers, conductive polymers, biodegradable polymers and so on.) with controlling their size ranging from several tens nm to several µm. By using this method, we fabricated particles with phase-separation structures from polymer blends or block-copolymers. Furthermore, it is shown that novel nanostructures including hemispherical particles and nano-disks by selective cross-linking and selective elution of polymer moieties that forms phase-separation structures are prepared.