Cyclotrimethylene trinitramine (RDX) with size of 0.8 to 2.6 mu m was produced by evaporation crystallization of RDX with ultrasonic spray. In the experimental range of the present work, the size of RDX crystals was found to be strongly affected by operating parameters including RDX concentration and furnace temperature. The size of RDX crystals was reduced as RDX concentration decreased and furnace temperature increased. The formation of random-shaped RDX particles due to agglomeration could be explained by coalescence between a crystal-containing droplet and a crystal or a crystal-containing droplet. In the present work, agglomeration was found to be controlled with injection of additives into starting solution: polyvinylpyrrolidone (PVP), which acted as a strong nucleation inhibitor in the crystallization of RDX from acetone, was very effective on the formation of spherical RDX particles. However, when the nucleation promoters such as polyoxyethylene 10 oleoyl ether (Brij 97) and oleylamine were added, highly agglomerated and random-shaped RDX particles were produced. Therefore, it can be concluded that the controlling onset time of nucleation is an important factor to improve crystal shape in evaporation crystallization by ultrasonic spray.