Monitoring of the cycle of crystal formation and dissolution gives important information for manipulating solute concentration and solution temperature for the preparation of mother solution in crystallization processes. In this study a direct monitoring technique using a quartz crystal sensor is applied to the crystallization of potassium bromide solution, and the monitoring performance is investigated. The experimental results indicate that the monitoring shows the detailed process of crystal formation and dissolution, and there is a hysteresis of 0.98 ℃ between the initiation temperature of crystal formation and the completion temperature of dissolution in the first measurement. In addition, it is demonstrated that the application of the technique is simple and efficient for the control of crystallization processes.