The basic assignment of this study was to investigate and compare the effect of mechanical mixing and ultrasonic irradiation on kinetics of borax nucleation in the batch cooling crystallizer. In the crystallizer with mechanical mixing, the experiments were performed at three different rotational speeds, while the experiments with ultrasound irradiation were performed at three different ultrasound amplitudes. The experiments were carried out in a laboratory scale crystallizer, with a volume of 2.14 dm 3 , in which the mother liquor was cooled down by linear cooling of 6 °C h -1 . In the first part of the work, for the applied impeller rotational speeds (N = 100 - 350 rpm) and ultrasound amplitudes (A = 20 – 40 %), the metastable zones widths of the mother liquors saturated in the temperature range of 20 to 35 ° C were determined. Based on the results of metastable zones widths at the examined process conditions, using the Mersmann's nucleation criterion the dominant nucleation mechanism was determined and the values of nucleation rates were calculated. Also, the values of power consumption per unit mass of suspension saturated at 30 °C, at examined conditions were determined and compared. On the basis of values of nucleation rates, the characteristics of the final crystallization products were assumed regarding crystals size. Deviations from the predicted assumptions are present in those systems where the state of complete suspension has not been achieved. Since that this is the case inside the all of the systems with ultrasound irradiation, it is suggested to conduct crystallization of borax at applied mixing conditions but also to examine the nucleation kinetics of borax in the systems in which mixing and ultrasound are combined.