In the drowning-out crystallization of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) with a spray, effects of operating parameters such as spray distance, solute concentration, and antisolvent temperature on crystal size were investigated. It was found that there exists an optimum spraying distance of solution for the production of submicrometer-sized crystals: the crystal size decreases with increasing spray distance in the distance shorter than the optimum distance due to incomplete atomization and that increases with the spray distance at the distance longer than the optimum distance due to coalescence among fluttered droplets. Higher RDX concentration in starting solution induced larger product particles. However, too low RDX concentration was also found to be not preferred for the small-sized product since nucleation occurs after complete mixing of atomized solution droplets with the antisolvent. By similar reason, it was found that lower antisolvent temperature, which causes faster nucleation, is preferred for the small-sized product. However, too low temperature was shown to be not preferred because of an increase in coalescence between droplets which leads to agglomeration. Finally, poly(vinylpyrrolidone) (PVP) was found to act as a nucleation promoter in the drowning-out crystallization with the acetone/water system, and RDX particles with an average size of 790 nm were successfully produced.