The purposes of this study were to investigate the effects of ultrasound irradiation on the number of crystals formed in an acetylsalicyclic acid crystallization process and to assess the controllability of the final product size via the number of primary nuclei. The number of crystals present after primary nucleation was counted and the relationship between the final product size and the number of crystals was examined. Additionally, the growing ASA crystals were observed, since ultrasound energy not only may control primary nucleation but may also the perfection of the crystal shape. At a high level of ultrasonic energy, ultrasound irradiation increased the average number of crystals, an effect that has been reported often; however, at a low level of ultrasonic energy it decreased the average number of crystals, and moreover, these opposing ultrasonic effects on the number of crystals interchanged at a specific energy threshold. These results reveal two novel phenomena-that there is an energy region where ultrasonic irradiation inhibits primary nucleation, and that a specific amount of ultrasonic energy is needed to activate primary nucleation. On the other hand, the final product size almost depended upon the number of primary nuclei, indicating that the final product size could be controlled via the number of crystals influenced by ultrasound irradiation. According to the photographs of crystals, they were not destroyed by the process. Therefore, it was proposed that ultrasound energy does not destroy the perfection of the crystal shape but only controls primary nucleation under the condition: both short irradiation time and low supersaturated condition. (c) 2005 Elsevier B.V. All rights reserved.