To obtain a unimodal crystal size distribution, linear cooling of a batch crystallizer was experimentally examined. Sodium sulfate, which has a relatively large heat of crystallization, was crystallized. The condition for batch crystallizer operation within the metastable region was derived : the product of temperature derivative of solute saturation concentration and cooling rate must be less than a certain value. Experiments were carried out with cooling rate satisfying the operation condition. A nearly unimodal product of sodium sulfate was obtained.