Abstract: For improvement of performance and yield of product, ultrafine particles of naphthalene are manufactured in this work under its solid and melting state by using rapid expansion of supercritical solution (RESS) process, repectively. Supercritical CO 2 is utilized as solvent for the process. Meanwhile, effects of extraction temperature, extraction pressure, preexpansion temperature and nozzle diameter on size and size distributions of product are investigated under solid and melting state of naphthalene. The results show, an increase in pre expansion temperature leads to an increase in particle size; an increase in extraction pressure leads to a decrease in particle size; changes in nozzle orifice diameter have almost no effect on particles for both solid and melting state. But an increase in extraction temperature revealed respectively a decrease and an increase in particle size for solid and melting state.