In the cooling type batch crystallization, the influence of undersaturation operation on crystal size distribution was investigated, and the effectiveness of this operation was discussed. In batch crystallization with seeding, it was clear that undersaturation operations in the early stages of the cooling operation was effective in dissolving the micro-crystals which were generated by the initial bleeding, whereas undersaturation operations halfway through cooling was effective in dissolving the micro-crystals which were generated by attrition. From the results of applying this cooling temperature program to non-seeded batch crystallization, the design strategy of the temperature program for non-seeded operation was able to be determined. And it became clear that the crystal size distribution was improved by using undersaturation operations also in the case of non-seeded operation. This strategy of undersaturation operations was confirmed from measurement of crystal size distribution using an in-line sensor.