The essential task of this research was to investigate the effect of simultaneous mixing and ultrasound impact on kinetics of borax nucleation in a batch crystallizer with controlled cooling. Experiments were done in a laboratory scale crystallizer, where the volume of mother liquor saturated at 30 °C was 2,65 dm 3 . The applied cooling speed was 6 °C h -1 . During the entire process, both mechanical stirring and ultrasound irradiation were simultaneously applied. All experiments were conducted at that impeller speed which ensured the state of complete suspension (N=N JS ), while the effect of four different ultrasound amplitudes, A=0, 20, 25 and 30%, was closely monitored. It was essential to determine the change of mother liquor concentration, which was done off-line by using the potentiometric method. Also, for all four ultrasound amplitudes applied, the change of absolute supersaturation during process time was monitored. Metastabile zone width was determined by using a widely accepted visual method. Based on that data, a dominant nucleation mechanism was determined by Mersmann’s nucleation criteria. Furthermore, for all experimental conditions tested, nucleation rates were calculated and compared. The characteristics of the final product of crystallization were analyzed in detail by sieving. In order to assess the energetic requirements of the experiment, the mechanical mixing power of the suspension as well as the power of ultrasound irradiation were determined and expressed as the power consumed per unit mass of suspension, P/m. Finally based on given results, the optimal process conditions for combined use of mehanical stirring and ultrasound irradiation were suggested.