The crystallization operation is widely used in chemical processes, and is one of unit operation which deals with crystallization phenomena. The purposes of crystallization operation are to separate desired component and to produce crystalline particles. However, phenomena of crystallization are not simple and the effects of operation conditions on product specification are not clear. The driving force of crystallization is supersaturation in non-equilibrium phenomena. So the operation strategy for designing supersaturation is important in order to keep high quality such as size distribution, crystal morphology and polymorph. In this paper, the relationships between supersaturation and crystal quality are discussed and the operation design methods for control crystal qualities are introduced.