A simple method to prepare block-copolymer nanoparticles and their unique phase-separation structures, were analyzed. The aqueous suspensions of nanoparticles of the block copolymers were prepared from their tetrahydrofuran (THF) solutions. The nanoparticles of the block copolymers can also be prepared by the conventional reprecipitation method. It is observed that THF gradually evaporates from the solution surface, and then a solvent gradient was formed from the bottom to the top of the solution. By this exposure to solvent, the well-developed microphase separation occurred in the block-copolymer nanoparticles. The results show that block copolymers formed a well-developed lamellar structure in the nanoparticles.