The crystallization is widely used in chemical processes, and is one of unit operation which deals with crystallization phenomena. The purposes of crystallization are to separate desired component and to produce crystalline particles. However, phenomena of crystallization are not simple and the relationships between operation conditions and product specification are complicated. The driving force of crystallization is supersaturation in non-equilibrium process. So the operation strategy for Jesigning supersaturation is important in order to keep high quality such as size distribution, cristal morphology and polymorph. In this paper, the relationships between supersaturation and crystal qualities are discussed, and the operation design methods are introduced to control crystal qualities in solution crystallization. (C) 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.